This Article Abstract Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Request Permissions Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Brette, F. Articles by Orchard, C. Search for Related Content PubMed PubMed Citation Articles by Brette, F. Articles by Orchard, C. Related Collections Pathophysiology Calcium cycling/excitation-contraction coupling Ion channels/membrane transport

(Circulation Research. 2003;92:1182.)
© 2003 American Heart Association, Inc.

Review

T-Tubule Function in Mammalian Cardiac Myocytes

Fabien Brette, Clive Orchard

From the School of Biomedical Sciences, University of Leeds, Leeds, UK.

Correspondence to Clive Orchard, School of Biomedical Sciences, University of Leeds, Leeds LS2 9JT, UK. E-mail c.h.orchard{at}leeds.ac.uk

	Abstract

Top Abstract Introduction Occurrence and Morphology of... Localization of Function Functional Role Changes During Development and... Conclusions and Unanswered... References

 
The transverse tubules (t-tubules) of mammalian cardiac ventricular myocytes are invaginations of the surface membrane. Recent studies have suggested that the structure and function of the t-tubules are more complex than previously believed; in particular, many of the proteins involved in cellular Ca²⁺ cycling appear to be concentrated at the t-tubule. Thus, the t-tubules are an important determinant of cardiac cell function, especially as the main site of excitation-contraction coupling, ensuring spatially and temporally synchronous Ca²⁺ release throughout the cell. Changes in t-tubule structure and protein expression occur during development and in heart failure, so that changes in the t-tubules may contribute to the functional changes observed in these conditions. The purpose of this review is to provide an overview of recent studies of t-tubule structure and function in cardiac myocytes.

Key Words: cardiac muscle • t-tubules • excitation-contraction coupling • heart failure

	Introduction

Top Abstract Introduction Occurrence and Morphology of... Localization of Function Functional Role Changes During Development and... Conclusions and Unanswered... References

 
The transverse tubules (t-tubules) of mammalian cardiac ventricular myocytes are invaginations of the surface membrane that occur at the Z line and have both transverse and longitudinal elements. Many of the proteins involved in excitation-contraction coupling appear to be concentrated at the t-tubules. Therefore, it has been suggested that the t-tubules play a central role in cell activation. In the present review, we will consider the immunohistochemical and functional evidence for protein localization at the t-tubules, potential problems in the interpretation of such data, and the functional consequences of such localization. We will also consider the possible role of the t-tubules in the functional changes that occur during cardiac development, hypertrophy, and failure.

	Occurrence and Morphology of the T-Tubules

Top Abstract Introduction Occurrence and Morphology of... Localization of Function Functional Role Changes During Development and... Conclusions and Unanswered... References

 
T-tubules are present in the cardiac tissue of all species of mammals so far investigated (eg, mice,¹ rats,² guinea pigs,³ rabbits,⁴ dogs,⁵ pigs,¹ and humans⁶) but appear to be absent in avian,⁷ reptile⁸ and amphibian⁸ cardiac tissue. Within mammalian cardiac tissue, t-tubules occur predominantly in ventricular myocytes, being either absent or far less developed in atrial, pacemaking, and conducting tissue,⁹ although a recent report has suggested that 50% of atrial myocytes possess a sparse irregular tubular system.¹⁰ The following discussion will concentrate on mammalian ventricular myocytes.

The t-tubules are invaginations of the sarcolemma and glycocalyx, which appears to remain associated with the sarcolemma within the t-tubules.¹¹ Early studies of cardiac muscle showed that they occur at the Z line, at the end of each sarcomere¹²; therefore, they occur at intervals of 2 µm along the longitudinal axis of the ventricular myocyte. Subsequent studies have shown that the t-tubular system also has longitudinal extensions.¹³ Although the t-tubules leave the surface membrane at the Z line, forming an approximately rectangular array, only 60% of the tubular volume occurs near the Z line; the other 40% occurs between the Z lines.² Thus, the t-tubular system is not, as its name might suggest, a simple transverse system of tubules but is a complex system of branching tubules with both transverse and longitudinal elements (Figure 1). Because of its complexity, it has been suggested that the transverse-axial tubular system,¹⁴ sarcolemmal Z rete,² or sarcolemmal tubule network¹¹ might be a more appropriate name. However, t-tubule remains the standard term and will be used in the present review, and surface sarcolemma will be used to describe that part of the cell membrane not within the t-tubules.

View larger version (152K): [in this window] [in a new window]   Figure 1. Three-dimensional structure of the t-tubular system in a living rat ventricular myocyte, reconstructed from a stack of images of dextran-linked fluorescein fluorescence within the t-tubules. Bar=5 µm. Reprinted from Soeller C, Cannell MB. Examination of the transverse tubular system in living cardiac rat myocytes by 2-photon microscopy and digital image–processing techniques. Circ Res. 1999;84:266–275, by permission of the American Heart Association ©1999.

Estimates of the percentage of the ventricular myocyte volume occupied by the t-tubules varies from 3.6% in rat myocytes² to 0.8% in mouse myocytes,⁷ although this variation probably reflects methodological rather than species differences because there appears to be no clear relationship between species and estimates of percentage cell volume occupied by the t-tubules. Similarly, estimates of the percentage of cell membrane located in the t-tubules vary from 64% (calculated in Bers¹¹ from data in Soeller and Cannell²) to 21%,¹⁵ with no apparent species differences.

The t-tubules of cardiac muscle have a mean diameter of 200 to 300 nm,¹³ although within a single rat ventricular myocyte, the diameter of individual tubules can vary from 20 to 450 nm, but with more than half the t-tubules having diameters between 180 and 280 nm.² One consequence of this network of narrow tubules is that a rapid change in the composition of the solution around a ventricular myocyte results in a slower change in the composition of the fluid within the t-tubules because of the time required for diffusion into the t-tubular network. In guinea pig myocytes, a rapid change of [Ca²⁺]_o results in the diffusion of Ca²⁺ into the t-tubules at 3 to 16 µm/s,¹⁶ so that the solution change within the t-tubules is delayed by up to 2.3 seconds, and washout of Ca²⁺ from the t-tubules occurs with a time constant of up to 1.7 seconds for the deeper regions of the t-tubular system,¹⁶ although these time courses may vary between species,¹⁷ possibly reflecting species differences in t-tubule morphology.

Although the diversity of estimates of the extent of the t-tubules within a species makes it impossible to determine whether there may be differences between species, it is tempting to speculate that the extent of the t-tubules varies between species depending on cell size (surface area/volume ratio) and heart rate if the t-tubules are necessary to produce synchronous Ca²⁺ release throughout the cell (see Coupling of Ca²⁺ Entry and Ca²⁺ Release, later) and, hence, synchronous contraction. Although we are unaware of any differences related to cell size, the density of the t-tubular network is greater in the mouse, which has a resting heart rate of 300 to 400 bpm, than in the pig, which has a heart rate of <100 bpm.¹

T-Tubule Development Is Labile
Ventricular myocytes isolated from neonatal hearts show little evidence of t-tubule development,^4,18 and cells kept in culture for 6 days show a progressive decrease of t-tubule density.¹⁹ T-tubule structure has also been reported to change in myocytes from failing hearts (see T-Tubule Development and Morphology in Hypertrophy and Failure). However, the mechanisms underlying the expression and maintenance of the t-tubules are not clear. Once formed, the ability of the tubular system to maintain its remarkable degree of structure despite the forces exerted during the normal contraction cycle may be due to the presence of a "scaffold" of focal adhesion molecules, membrane-associated proteins, and basal lamina proteins.²⁰

Proteins Present in the T-Tubule Membrane
The function of the t-tubules depends not only on their structure but also on the proteins within, and adjacent to, the t-tubule membrane. Immunohistochemical techniques have been widely used to investigate the location of proteins within cardiac ventricular myocytes. Such studies have shown marked variations in the distribution of membrane proteins, although a distinction needs to be drawn between the fraction of a particular type of protein located at the t-tubule and whether this is greater than the fraction of the total cell membrane located within the t-tubules, ie, whether protein density is higher (concentrated) at the t-tubule. This is discussed further in Interpretation of Immunohistochemical Data and in Conclusions and Unanswered Questions.

Ca²⁺-Handling Proteins
The location of sarcolemmal Ca²⁺-handling proteins is important because of their role in excitation-contraction coupling and because the Ca²⁺-release channels (ryanodine receptors [RyRs]) of the sarcoplasmic reticulum (SR) are concentrated close to the t-tubule (see Protein Colocalization Within, and With Proteins Adjacent to, the T-Tubules).

In the rabbit heart, an early study showed that a membrane fraction from the t-tubules had a higher density of L-type Ca²⁺ channels than did membrane from the surface sarcolemma.²¹ A more recent immunologic study also found this channel to be concentrated in the t-tubules of adult rabbit ventricular myocytes, with less staining of the surface sarcolemma, although the surface staining that did occur was punctate and associated with junctional SR.²² In rat heart, the L-type Ca²⁺ channel, and hence current (I_Ca), is also concentrated in the t-tubules, with estimates ranging from 3 to 9 times more concentrated in the t-tubule membrane than on the surface sarcolemma.^23–26 Comparative studies suggest that the t-tubular concentration of the L-type Ca²⁺ channel is greater in rat ventricular myocytes than in those from the rabbit.²²

Although a high density of L-type Ca²⁺ channel at the Z line has been reported in all studies of which we are aware, it has also been reported that in some rat ventricular myocytes, the channel is present in appreciable amounts on the surface membrane,²⁵ and in sheep and bovine cardiac tissue, it has been reported that this channel is present at a lower concentration on the putative t-tubule membrane than on the surface sarcolemma.²⁷ However, the majority of recent reports suggest that this channel is concentrated in the t-tubules compared with the surface sarcolemma.

The distribution of the Na⁺-Ca²⁺ exchanger (NCX) has been more controversial. The first study of its distribution showed it to be predominantly in the t-tubules in guinea pig ventricular myocytes.²⁸ Subsequent studies (eg, Musa et al²⁵ and Kieval et al²⁹) have shown a more even distribution between the t-tubule and surface membranes, although a recent immunologic study of rat ventricular myocytes also showed NCX located predominantly within the t-tubular network.²⁶ Thus, although there is a consensus that the NCX protein is found in the t-tubule membrane, it is less clear whether this protein is at a higher concentration in the t-tubules or is homogeneously distributed between the t-tubules and surface sarcolemma.

The distribution of other important sarcolemmal Ca²⁺-handling proteins, in particular, the sarcolemmal Ca²⁺-ATPase and the T-type Ca²⁺ channel, is currently unknown.

Na⁺-Handling Proteins
In addition to NCX, other proteins that allow Na⁺ to cross the cell membrane also show different distributions between the t-tubules and surface sarcolemma. For example, the Na⁺-H⁺ exchanger, which regulates intracellular pH by extruding H⁺ from the cell, is concentrated at the t-tubules and intercalated disks.³⁰ Immunolabeling of the Na⁺,K⁺-ATPase has shown that different isoforms are differently distributed, with the ₁ isoform being concentrated in the t-tubules of rat ventricular myocytes and the ₂ isoform showing a more homogeneous distribution.³¹ In contrast, in guinea pig myocytes, which only express the ₁ isoform, this isoform showed a more uniform distribution between the t-tubule and surface sarcolemma.³¹

The Na⁺ channel has been found to be localized to the t-tubules in ventricular myocytes (see Scriven et al²⁶). A recent immunologic study has shown that the cardiac ventricular myocyte expresses several different Na⁺ channel isoforms, with the "cardiac" pore-forming subunit Na_v1.5 located predominantly at the intercalated disks but with the "brain" isoforms Na_v1.1, Na_v1.3, and Na_v1.6 expressed predominantly in the t-tubules.³² However, the function of these isoforms is unclear, because their high tetrodotoxin sensitivity (nanomolar) is in contrast with the high concentrations of tetrodotoxin (micromolar) required to affect cardiac preparations.¹¹

Thus, the t-tubules may represent a region of the cell where the regulation of Na⁺ is different from that of the surface sarcolemma, although the differences appear less marked than for Ca²⁺-handling proteins. Changes in [Na⁺]_i, by altering the activity of NCX, may have marked effects on [Ca²⁺]_i and, hence, contractility, although this may also depend on the proximity of the Na⁺-handling proteins to NCX within the t-tubule (see Protein Colocalization Within, and With Proteins Adjacent to, the T-Tubules).

K⁺-Handling Proteins
There are relatively few studies of the distribution of proteins carrying K⁺ ions. Barry et al³³ observed K_v4.2, K_v1.2, K_v1.5, and K_v2.1 concentrated at the surface membrane of rat ventricular myocytes, with labeling intensity highest at the intercalated disk. However, as pointed out in Takeuchi et al,³⁴ the resolution of staining in that study was relatively low; subsequently, K_v4.2, one of the channels that underlies the transient outward current (I_to) in ventricular myocytes, was shown to be localized predominantly to the t-tubular system. Similarly, the K⁺ channel TASK-1,³⁵ which may underlie the steady-state outward current (I_ss), and K_ir2.1,³⁶ which is believed to underlie the inward rectifier K⁺ current (I_K1), also appear to be predominantly in the t-tubules.

Anion-Handling Proteins
In addition to Na⁺-H⁺ exchange, another pH-regulating protein, the Cl^--HCO₃^- exchanger, is present, and may be concentrated, at the t-tubule.³⁷ It is tempting to speculate that it is important that the pH of this region of the cell, which contains so many proteins important for normal cell function, is tightly regulated.³⁰

Second-Messenger Pathways
The ß-adrenergic pathway is important in the regulation of a number of key proteins in the heart, in particular, the L-type Ca²⁺ channel, the SR Ca²⁺-ATPase (via the regulatory protein phospholamban), and the contractile proteins (via troponin I). It has been suggested that there is local regulation by this pathway, particularly of the L-type Ca²⁺ channel.³⁸ A structural basis for such regulation is suggested by reports that key elements in this signaling cascade (G_s, adenylate cyclase, and A-kinase anchoring protein) are concentrated at the t-tubule membrane.^39–41 Interestingly, the protein phosphatase calcineurin, which will antagonize the effects of this pathway, appears to be colocalized with protein kinase A (PKA) near the t-tubules.⁴² The ß₂-receptor may also be associated with the L-type Ca²⁺ channel, because in neurons this receptor has been shown to be part of a macromolecular signaling complex with the L-type Ca²⁺ channel Ca_v1.2, a G protein, and adenylate cyclase.⁴³ This could account for the observation that stimulation of this receptor can increase I_Ca without an increase in whole-cell cAMP concentration (see Signal Transduction). Such a localized response has also been shown for the NO synthase 3 pathway.⁴⁴

Interpretation of Immunohistochemical Data
Data from these studies show many key proteins concentrated at the t-tubule (Figure 2). However, there are a number of potential problems with immunohistochemical studies. First, the observed distributions may be artifactual. It has, for example, been suggested that high-intensity staining at the intercalated disks and t-tubules may be a consequence of membrane folding and a high "brightness factor," respectively, so that quantification of protein distribution from such studies is difficult.⁴⁵ Second, the observed distribution may depend on epitope accessibility rather than protein distribution,⁴⁶ and spatial resolution is limited; further immunogold labeling may yield more information. Third, it is not always clear that a "striated" pattern of cell staining is colocalized with the t-tubule membrane or that the protein is inserted in the membrane. Therefore, it is difficult to be sure that particular proteins are concentrated at the t-tubules, particularly when the amount of membrane within the t-tubules is also unclear.

View larger version (36K): [in this window] [in a new window]   Figure 2. Schematic diagram of protein distribution between the t-tubules and surface sarcolemma. See text for further explanation.

It is interesting, however, that so many proteins are identified at the Z line, which, if correct, raises questions of whether these proteins function at the t-tubules or whether they are simply trafficked via this region of the cell. It is not clear that protein location always corresponds to function, partly because of some of the problems mentioned above and partly because protein function will also depend on other factors, such as local environment, second messengers, and accessory proteins. Localization of protein function will be considered further (in Localization of Function). Concentration of many proteins at the t-tubules may reflect their importance in excitation-contraction coupling (see Coupling of Ca²⁺ Entry and Ca²⁺ Release), although this will depend on not just the presence of the proteins but their juxtaposition (see Protein Colocalization Within, and With Proteins Adjacent to, the T-Tubules, below).

Protein Colocalization Within, and With Proteins Adjacent to, the T-Tubules
The SR is the major intracellular Ca²⁺ store in cardiac muscle. Ca²⁺ entry across the cell membrane triggers the release of further Ca²⁺ from the SR, via the RyR,⁴⁷ and it is predominantly this Ca²⁺ that causes contraction.

RyRs were first reported as "feet"⁴⁸ between the sarcolemma and SR membrane. Immunologic studies have shown a high density of RyRs in the junctional SR (the Ca²⁺-release site) adjacent to the t-tubule.^23,26 Thus, the site of Ca²⁺ release from the SR appears to be concentrated at the t-tubule, adjacent to the site of transsarcolemmal Ca²⁺ flux (see Ca²⁺-Handling Proteins).

Interestingly, the SR Ca²⁺ uptake pump (SERCA2), which is responsible for removing Ca²⁺ from the cell cytoplasm to cause relaxation and which, according to structural and biochemical studies (eg, Jorgensen et al⁴⁹), is thought to be located throughout the SR membrane, has been shown immunohistochemically to be concentrated at the Z line, adjacent to the t-tubule.²⁵ The regulatory protein phospholamban, which modulates the activity of SERCA2, shows a similar distribution (F. Brette, unpublished data, 2002).

Immunologic investigation of colocalization of proteins involved in excitation-contraction coupling has shown that Ca²⁺ channels are highly colocalized with RyRs in the t-tubules, forming the dyad.^23,26 However, NCX shows little colocalization with either the RyR or Na⁺ channel.²⁶ Therefore, it has been suggested that the t-tubule can be subdivided into 3 domains²⁶: (1) the dyad, (2) one containing the Na⁺ channel, and (3) one containing NCX. This suggests that the most effective functional coupling will be between the L-type Ca²⁺channel and the RyR, although the mechanism of colocalization is still unknown. The lack of colocalization of NCX with RyR may be one reason why Ca²⁺ influx via NCX is a less effective trigger for SR Ca²⁺ release.^50,51 The lack of colocalization of NCX and Na⁺ channels is also interesting, because it has been suggested that Na⁺ influx via Na⁺ channels may enhance Ca²⁺ entry on the exchanger,⁵² a mechanism that would presumably require colocalization; however, the proximity of NCX to junctional SR may allow NCX activity to be modulated by Ca²⁺ released from the SR (see Trafford et al⁵³ and Ca²⁺ Efflux).

	Localization of Function

Top Abstract Introduction Occurrence and Morphology of... Localization of Function Functional Role Changes During Development and... Conclusions and Unanswered... References

 
The studies described above suggest that many membrane proteins are concentrated at the t-tubules. However, protein function will not necessarily reflect protein distribution (above). A different approach has been to investigate the localization of function by using a number of techniques, which follow.

Cells Lacking T-Tubules
Some cardiac cells do not have a t-tubular system or have only a very sparse system; these include ventricular myocytes from embryonic and newborn animals, atrial cells, and Purkinje cells.^4,9,54,55 In addition, adult ventricular myocytes kept in culture lose their t-tubular system.^19,56 A number of studies have investigated the function of these cell types, which may help our understanding of t-tubule function.

In myocytes lacking t-tubules (from the ventricles of newborn rabbits, atrial cells, rabbit Purkinje cells, and cultured ventricular myocytes), electrical stimulation causes a rise of [Ca²⁺]_i that occurs initially at the cell periphery and propagates into the cell interior by propagated Ca²⁺-induced Ca²⁺ release in atrial⁵⁵ and cultured⁵⁶ cells and by diffusion in neonatal⁴ and Purkinje⁵⁴ cells, and localized Ca²⁺-release events (Ca²⁺ sparks) occur predominantly close to the surface sarcolemma in these cells.⁵⁶ This is in contrast to adult ventricular myocytes, in which Ca²⁺ release occurs synchronously across the width of the cell on electrical stimulation,^4,57,58 and Ca²⁺ sparks arise predominantly at the t-tubule.⁵⁹ These data are consistent with the notion that t-tubules play an important role in causing synchronous Ca²⁺ release in the adult ventricular myocyte, because of the concentration of L-type Ca²⁺ channels and RyR at the t-tubule (see Ca²⁺-Handling Proteins, above), although a problem with this type of study for the investigation of t-tubule function is that protein expression, as well as t-tubule structure, may be different in these cells.

An alternative approach has been to correlate the loss of t-tubules with loss of membrane currents. During 4 days in culture, the membrane capacitance of rabbit ventricular myocytes, taken to represent membrane area, decreases by 51% with a time course that correlates with the loss of 83% of I_K1 and 88% of the ATP-sensitive K⁺ current (I_K,ATP),⁶⁰ suggesting that these currents are concentrated in the t-tubules. However, another study has shown more complicated changes in rabbit ventricular myocytes during culture¹⁹: although t-tubule density and cell capacitance decrease continuously during 6 days in culture, Ca²⁺ channel density decreases by 50% after 1 day and then partially recovers, I_K1 also declines by 50% within 1 day but shows no recovery, whereas I_to changes little in 1 day but decreases by 65% after 6 days, emphasizing the difficulty of distinguishing between changes due to loss of the t-tubules and changes in protein expression.

Diffusion Studies
A different approach has been to investigate the rate of change of membrane currents in ventricular myocytes after a rapid change in perfusate composition. Because of the long diffusion time into the t-tubules (see Occurrence and Morphology of the T-Tubules), currents on the cell surface would be expected to change rapidly, whereas those located in the t-tubules would be expected to change over a longer time course. By use of this technique, it has been shown that the time course of K⁺ current inhibition by Ba²⁺ is consistent with a model that includes a diffusion time constant of 300 ms, consistent with much of I_K1 and I_K,ATP being localized in the t-tubules.⁶⁰ A similar approach has been used in guinea pig myocytes to investigate the time course of changes of the Na⁺ current (I_Na) and I_Ca in response to rapid changes of extracellular Na⁺ and Ca²⁺, respectively.⁶¹ In atrial cells, which lack t-tubules, changing the bathing Na⁺ or Ca²⁺ produced rapid changes of I_Na and I_Ca (time constant of 25 ms). However, in ventricular myocytes, only 36% of the current changed rapidly; the remaining 64% of current changed with a time constant of 200 ms, suggesting that this percentage of I_Na and I_Ca is within the t-tubules. Interestingly, this percentage is the same as the upper estimate of the percentage of cell membrane within the t-tubules (see Occurrence and Morphology of the T-Tubules), which would imply that these currents are found predominantly, but not concentrated, within the t-tubules.

Scanning Pipette
An elegant technique has been described recently that uses a pipette to scan the cell surface and simultaneously monitor membrane currents.⁶² To date, this technique has been used to investigate the location of I_K,ATP, which appears to be concentrated in the vicinity of the Z line, consistent with the proposal (see Diffusion Studies) that this current is found predominantly in the t-tubule. However, it is not clear whether the current monitored at the Z line by this technique simply reflects the amount of membrane under the electrode (ie, in the t-tubule) or whether the current is concentrated in the t-tubule or concentrated on the surface sarcolemma at the Z line.

Detubulation
Another approach developed recently has been to adapt the "osmotic shock" technique used previously to detubulate skeletal muscle to disrupt the t-tubules of rat ventricular myocytes.²⁴ Detubulation decreases cell capacitance by 30%.²⁴ It is not clear that the standard method used to measure membrane capacitance (eg, 10-mV 10-ms hyperpolarizing pulses from -80 mV) monitors the capacitance of all the t-tubule membrane (see Electrical Properties of T-Tubules). However, assuming the capacitance of the t-tubule membrane to be the same as that of the surface sarcolemma, this gives a lower limit to the percentage of the cell membrane found within the t-tubules.

Most (87%) of I_Ca and almost all NCX activity are lost after detubulation,^24,58 suggesting that the function of both of these Ca²⁺ flux pathways is concentrated in the t-tubules of rat ventricular myocytes, although if cell capacitance is underestimated more than ionic currents, it is possible that these currents are concentrated less than these measurements might suggest (see Electrical Properties of T-Tubules). In contrast, I_Na shows uniform distribution between the t-tubule and surface membranes,⁵⁸ as do I_K, I_to, and I_K1, although I_ss appears to be concentrated in the t-tubules.⁶³ This distribution of I_ss agrees with immunohistochemical studies of TASK-1 (see K⁺-Handling Proteins). However, the proteins thought to underlie I_to and I_K1 appear to be concentrated in the t-tubules (see K⁺-Handling Proteins); this discrepancy may be due to the difficulties of these techniques (see Interpretation of Immunohistochemical Data, above, and Electrical Properties of T-Tubules, below) or to local regulation or differential distribution of accessory proteins such as K_v4.3,⁶³ which, with K_v4.2, forms functional I_to channels.

Detubulation shows a greater percentage of Ca²⁺ flux via I_Ca and NCX in the t-tubules than might be expected from immunohistochemical studies. One way of reconciling these data is to suggest that the protein in the t-tubule is more active than that in the surface membrane (because of stimulation of I_Ca by second-messenger pathways, for example), so that relatively more function is lost than protein after detubulation. In support of this idea, ß-adrenergic stimulation of I_Ca causes a greater increase in current in normal cells than in detubulated cells,⁶⁴ suggesting that I_Ca in the t-tubules might be better coupled to this second-messenger pathway than that in the surface membrane. In the presence of tonic activity of this pathway, this could explain the relatively large loss of current after detubulation.

	Functional Role

Top Abstract Introduction Occurrence and Morphology of... Localization of Function Functional Role Changes During Development and... Conclusions and Unanswered... References

 
Electrical Properties of T-Tubules
Localized depolarization of skeletal muscle at the t-tubule causes contraction of the adjacent half sarcomeres.⁶⁵ By analogy, it has been assumed that the t-tubules in cardiac myocytes allow propagation of excitation into the cell to cause synchronous activation, although analogous experiments in cardiac muscle have not shown localized activation, possibly because the longitudinal extensions of the t-tubules allow the spread of excitation to adjacent regions of the cell.⁶⁶

Tidball et al⁶⁷ observed that the response of the action potential of rabbit ventricular trabeculae to the SR inhibitor ryanodine was correlated with the degree of t-tubule development and postulated that the t-tubules had a density of Ca²⁺ and K⁺ channels that was different from that of the surface membrane. It now appears likely that there are at least 3 factors that could contribute to differences between the t-tubule and surface membrane action potentials: (1) many of the membrane proteins that underlie the action potential are unevenly distributed between the surface and t-tubule membranes (above), so that the electrophysiology of the t-tubules would be expected to differ from that of the surface membrane; (2) modeling of skeletal muscle t-tubules has shown that 2 to 3 ms is required before the potential throughout a simple t-tubule model matches that at the cell surface,⁶⁸ and the complex structure of the cardiac t-tubular system may result in longer delays; and (3) the t-tubules form a restricted extracellular space that will allow ion accumulation and depletion, which can modify electrical activity (see Clark et al³⁶). In isolated cells, this may differentially alter the t-tubule action potential; in the intact muscle, diffusion is also restricted in the intercellular clefts, and the extent to which diffusion from the t-tubules and the intercellular clefts will differ is unknown.

Consideration of the electrical properties of the t-tubules is important (1) because it seems likely that the t-tubules are the most important site for excitation-contraction coupling and (2) because standard voltage-clamp techniques may not effectively clamp potential, and hence monitor membrane currents, within the t-tubular system.⁶⁹ Thus, ionic currents at the surface membrane may be more effectively monitored than those in the t-tubules, so that loss of currents with loss of t-tubules (see Cells Lacking T-Tubules and Detubulation) may be underestimated. Similarly, since the voltage drop down the t-tubules is unknown, it is possible that measurements of cell capacitance (see Cells Lacking T-Tubules and Detubulation) underestimate the percentage of cell membrane within the t-tubules. If capacitance and membrane currents are underestimated to a similar degree, calculation of the concentration of these currents in the t-tubules will be the same. However, if cell capacitance is underestimated more than ionic currents, these currents may be less concentrated in the t-tubules than suggested by such measurements.

Coupling of Ca²⁺ Entry and Ca²⁺ Release
The data presented above suggest that I_Ca and NCX activity occur predominantly in the t-tubules, close to the RyR, implying that the t-tubules play a central role in Ca²⁺ cycling and excitation-contraction coupling in cardiac ventricular myocytes. Although I_Ca is more effective than NCX in triggering SR Ca²⁺ release,^50,51 probably because the L-type Ca²⁺ channel, unlike NCX, is colocalized with RyR²⁶ and because of the higher Ca²⁺ flux through the channel,⁷⁰ the presence of NCX in the t-tubules means that Ca²⁺ flux via NCX may participate in the process.⁷¹

The local control theory of Ca²⁺ release (see Wier and Balke⁴⁷ for review) is that local Ca²⁺ entry across the cell membrane, predominantly via I_Ca, triggers local Ca²⁺ release from an adjacent cluster of RyRs. The whole-cell Ca²⁺ transient is the temporal and spatial sum of these individual localized release events.⁷² Ca²⁺ sparks occur predominantly near the t-tubules,^59,73 and localized Ca²⁺ release (Ca²⁺ spikes) occurs at discrete sites at the Z line.⁷⁴ These spikes (Figure 3) are proportional to I_Ca and to derived SR Ca²⁺ flux, providing strong support for the idea that local Ca²⁺ entry across the t-tubule membrane triggers local Ca²⁺ release from adjacent RyRs.

View larger version (41K): [in this window] [in a new window]   Figure 3. Visualization of t-tubule–SR dyadic release. A, Confocal line-scan image showing discrete t-tubule–SR Ca2+-release events (black arrowheads, top), mean fluorescence (middle), and ICa (bottom) on depolarization as shown above traces. Bar=5 µm. B, Data from panel A shown as a 3D plot of fluorescence (F) in time (t) and space (X). Modified from Song LS, Sham JS, Stern MD, Lakatta EG, Cheng H. Direct measurement of SR release flux by tracking "Ca2+ spikes" in rat cardiac myocytes. J Physiol. 1998;512:677–691, by permission of the Physiological Society ©1998.

The importance of the Ca²⁺ release occurring at the t-tubules has been demonstrated in cells lacking t-tubules in which the electrically stimulated rise of [Ca²⁺]_i initially occurs close to the cell membrane and then either diffuses (Purkinje⁵⁴ and neonatal⁴ cells) or propagates via Ca²⁺-induced Ca²⁺ release (atrial,⁷⁵ cultured,⁵⁶ and detubulated^57,58 cells) into the cell (Figure 4). Interestingly, in detubulated cells, the speed of propagation is increased by ß-adrenergic stimulation⁷⁶ so that synchronization of Ca²⁺ release is increased⁷⁷ even in the absence of t-tubules.⁷⁶ In addition, compared with pig myocytes, mouse myocytes show a rapid and synchronous rise of [Ca²⁺]_i.¹ This difference appears to be due to the higher t-tubule density in mouse myocytes. Therefore, it appears that Ca²⁺ release occurs predominantly at the t-tubules, which therefore underlie the temporally and spatially synchronous Ca²⁺ release observed in ventricular myocytes (see Yang et al⁵⁸).

View larger version (24K): [in this window] [in a new window]   Figure 4. Spatial [Ca2+]i gradients in cardiac myocytes. Transverse line scans show that [Ca2+]i rises synchronously across the cell width in a ventricular cell after electrical stimulation (S), whereas in cells lacking t-tubules (detubulated, atrial, and Purkinje), [Ca2+]i rises initially at the cell edge and then propagates into the cell center. Scales have been changed to allow qualitative comparison. Modified from Cordeiro JM, Spitzer KW, Giles WR, Ershler PE, Cannell MB, Bridge JH. Location of the initiation site of calcium transients and sparks in rabbit heart Purkinje cells. J Physiol. 2001;531:301–314; Yang Z, Pascarel C, Steele DS, Komukai K, Brette F, Orchard CH. Na+-Ca2+ exchange activity is localized in the t-tubules of rat ventricular myocytes. Circ Res. 2002;91:315–322; and Blatter LA, Kockskamper J, Sheehan KA, Zima AV, Huser J, Lipsius SL. Local calcium gradients during excitation-contraction coupling and alternans in atrial myocytes. J Physiol. 2003;546:19–31, by permission.

Although NCX is well recognized as an important Ca²⁺ efflux pathway (see Ca²⁺ Efflux), the suggestion that Ca²⁺ entry on NCX at the start of the action potential might trigger SR Ca²⁺ release⁷⁸ has been more controversial; most studies suggest that it is not as effective as I_Ca (above) and may not be the normal physiological trigger but rather modulates the trigger effect of I_Ca and/or acts as a trigger only under certain conditions. It has been proposed that Ca²⁺ entry on the exchanger, and hence its ability to act as a trigger, is enhanced by colocalization with I_Na, which will increase the [Na⁺] in a submembrane microdomain (fuzzy space⁷⁹) that is sensed by NCX. However, Scriven et al²⁶ have shown that the L-type Ca²⁺ channel, NCX, and I_Na are in different microdomains, although a demonstration that brain-specific isoforms of the Na⁺ channel are concentrated in the t-tubules³² may require the colocalization of these channels with other proteins in the t-tubule to be investigated.

As well as "feed-forward" from Ca²⁺ entry via L-type Ca²⁺ channels and NCX to the RyR, the proximity of these proteins suggests that Ca²⁺ released from the SR may "feed back" onto the sarcolemmal proteins concentrated at the t-tubule (see Ca²⁺ Efflux, below).

Ca²⁺ Efflux
There are 2 sarcolemmal Ca²⁺ efflux pathways in ventricular myocytes: NCX and sarcolemmal Ca²⁺-ATPase. The exchanger appears to be localized within the t-tubules and is therefore close to the site of SR Ca²⁺ release (above); the distribution of Ca²⁺-ATPase is unknown, although it is present on the surface membrane.⁵⁸ There are at least 2 lines of evidence to support the idea that Ca²⁺ released from the SR has "privileged" access to NCX. First, when Ca²⁺ is released from the SR using caffeine, there is hysteresis between bulk [Ca²⁺]_i and NCX current, with a given current being produced by a lower bulk [Ca²⁺]_i as [Ca²⁺]_i is increasing, suggesting that Ca²⁺ is released close to NCX, resulting in a higher local [Ca²⁺]_i that stimulates NCX.⁵³ Second, Ca²⁺ efflux appears to occur during systole, compatible with Ca²⁺ release occurring close to NCX.⁸⁰ Interestingly, recent immunohistochemical work has shown that SR Ca²⁺-ATPase also appears to be localized close to the t-tubule.²⁵

The observation that major Ca²⁺ sequestration pathways are located at the t-tubule raises the question of why this is so. The presence of NCX in the t-tubules may allow rapid Ca²⁺ efflux throughout the cell, thus helping to produce synchronous relaxation. However, it also appears that this arrangement will produce futile Ca²⁺ cycling. The reason for this is unknown, although it has been suggested that by regulating [Ca²⁺] close to the RyR, NCX may alter the threshold for, and thus help regulate, SR Ca²⁺ release.⁷¹

Signal Transduction
Colocalization of PKA and calcineurin occurs at the t-tubules (see Second-Messenger Pathways), and a local increase in cAMP along the Z lines has been shown by using fluorescence resonance energy transfer (Zaccolo and Pozzan⁸¹). These authors suggested that a localized increase of cAMP occurs near the t-tubules, although their preparation, embryonic cardiac cells, lacks t-tubules. However, this suggests that the Z line, rather than the t-tubules, might provide the scaffold for the cell structure in this region, and further experiments in adult ventricular myocytes would be of interest.

There is also functional evidence for a local elevation of cAMP and subsequent protein phosphorylation. Xiao⁸² has provided evidence that ß₂-adrenergic stimulation can produce an increase in I_Ca without causing phosphorylation of other proteins, such as phospholamban, compatible with local signaling and close association of the ß₂ signaling pathway with the L-type Ca²⁺ channel (see Second-Messenger Pathways). The observation that the L-type Ca²⁺ channel is less sensitive to ß-adrenergic stimulation in detubulated cells, although phospholamban phosphorylation is unaffected,⁸³ suggests that this local signaling occurs predominantly in the t-tubules.

	Changes During Development and Disease

Top Abstract Introduction Occurrence and Morphology of... Localization of Function Functional Role Changes During Development and... Conclusions and Unanswered... References

 
Data from normal cells suggest that the major difference between the t-tubule and surface membranes is the concentration of Ca²⁺-handling proteins at the t-tubules. Therefore, in this section, we will concentrate on these proteins.

T-Tubule Development and Morphology in Hypertrophy and Failure
Embryonic and neonatal cardiac myocytes lack t-tubules,^4,84 which develop during the first few weeks of life, with the precise time of development differing between species. In addition, during cell culture (above) and heart failure (below), t-tubule morphology changes, indicating that t-tubule structure is labile even in the mature cell.

How the t-tubules develop and are maintained is less clear. Lee et al⁸⁵ have shown that expression of amphisin-2, a protein that can link the plasma membrane and submembranous cytosolic scaffolds in CHO cells, generates narrow tubules that are continuous with the plasma membrane. The t-tubule membrane appears to have a distinct protein and lipid composition and is enriched in cholesterol, which can be used as a tool to separate t-tubule and surface membranes.⁸⁶ The development of the t-tubules appears to depend on protein and lipid and shows properties that are similar to the development of caveolae, which requires cholesterol and caveolin-3.⁸⁷ T-tubules are composed of interconnected caveolae-like elements, and repeated caveolae formation in the absence of fission leads to the generation of t-tubules.^88,89 Treatment with amphotericin B, an antibiotic that binds cholesterol, causes the disruption of t-tubules in C₂C₁₂ myotubes.⁸⁶ This does not exclude the existence of "true" caveolae (50- to 80-nm-diameter invaginations, see Razani et al⁸⁷ for review) along the surface sarcolemma and t-tubule membrane.¹¹

Despite the evidence that the t-tubules are important in excitation-contraction coupling, there have been relatively few studies of this network during pathological conditions. An early morphological study showed that hypertrophy of rat left ventricle, produced by aortic constriction, results in an increase in the t-tubule area, which helps maintain the surface area/volume ratio of the hypertrophied cells.⁹⁰ However, in rat doxorubicin-induced cardiomyopathy, cell capacitance decreased significantly, possibly as a result of t-tubule damage.⁹¹ To date, only one study has examined t-tubule structure in pathological living cells. He et al⁵ showed that in canine tachycardia-dilated cardiomyopathy, t-tubules are lost at each extremity of the cell but remain intact in the center of the cell (interestingly, a similar pattern of t-tubule loss has been reported during culture¹⁹); however, cell capacitance increased by 13%, possibly because of a hypertrophy-induced increase in surface sarcolemma compensating for the loss of t-tubules.

Data from the human heart is equally equivocal. In hypertrophic human heart, the t-tubules appear to be aberrantly shaped⁹² or dilated.²⁰ Dilation has been also observed in failing human heart,⁹³ although the changes were not quantified in either study. In contrast, a preliminary study of human ventricular myocytes suggests that t-tubule density is not altered in failing hearts,⁹⁴ whereas another preliminary report showed decreased t-tubule density in myocytes from failing human ventricle.⁶

The diversity of results from animal and human studies may reflect the diversity of models and conditions studied. Further investigation of the development, morphology, and composition of the t-tubules from the failing heart may elucidate both the regulation of t-tubule structure^5,92 and its role in the changes in function observed in pathological conditions (see Gomez and colleagues^95,96 and Relevance of Changes in T-Tubule Structure and Protein Expression to Function).

Changes in Protein Expression in Development, Hypertrophy, and Failure
Ventricular myocytes from newborn animals show little t-tubule development (see T-Tubule Development and Morphology in Hypertrophy and Failure) and relatively little SR⁹⁷ but enhanced I_Ca⁹⁸ and NCX activity⁹⁹ compared with adult cells. It seems likely that these cells rely mainly on extracellular Ca²⁺ for activation.¹⁰⁰ The transition to the more mature (SR-dominated) form of excitation-contraction coupling during the first few weeks of life is accompanied by SR development, decreased I_Ca,⁹⁸ and t-tubule formation (Haddock et al⁴ and Chen et al¹⁸), and NCX at least has been reported to appear in the t-tubules as soon as they are formed.¹⁸

Ventricular myocytes from failing hearts show reduced contraction, blunted ß-adrenergic responsiveness, and myocyte hypertrophy. For review, see Tomaselli and Marban¹⁰¹; we will concentrate on changes in the proteins that occur at the t-tubules.

A decrease in the amplitude and the rate of decline of the systolic Ca²⁺ transient is a consistent finding in failing heart muscle. This appears to be due to depressed SERCA2 activity, which will slow the rate of decline of the Ca²⁺ transient and decrease SR Ca²⁺ content. In addition, RyRs may be hyperphosphorylated, increasing Ca²⁺ leak¹⁰² and reducing SR Ca²⁺ content.

Most reports show no change in the density of I_Ca during hypertrophy and failure.¹⁰³ This may be due to the presence of fewer channels, with upregulation of the remaining channels,^5,104 consistent with work showing an increase in the activity of single channels from failing human ventricular myocytes.¹⁰⁵ This upregulation may be due to channel phosphorylation,¹⁰⁴ which may also explain, at least in part, the blunted response of failing hearts to ß-adrenergic stimulation.

NCX expression and current density appear to be increased in hypertrophy and failure,¹⁰⁶ although it has recently been shown that Ca²⁺ transport via NCX is virtually unchanged in rat ventricular myocytes after myocardial infarction if NCX function is normalized to cell volume.¹⁰⁷

Thus, there have been many studies of the changes that occur in the hypertrophied and failing heart and, in particular, of proteins that are normally found at the t-tubules. There is also evidence that the t-tubule structure changes in these conditions. However, there is a paucity of data regarding whether these changes are associated with changes in the distribution of proteins. It is tempting to speculate that remodeling of the t-tubules and changes in Ca²⁺-handling protein expression and distribution might alter excitation-contraction coupling in failing ventricular myocytes.

Relevance of Changes in T-Tubule Structure and Protein Expression to Function
Myocytes from newborn animals have a relatively large surface area/volume ratio,⁹⁹ and myofilaments are located in the subsarcolemmal region.⁸⁴ It appears likely that sufficient Ca²⁺ can enter via the surface membrane to activate the myofilaments. However, these cells also show marked gradients of Ca²⁺ during excitation-contraction coupling: Ca²⁺ rises initially at the cell periphery and then diffuses into the center of the cell.⁴ The development of the t-tubules is associated with a transition to the mature pattern of excitation-contraction coupling, which shows a synchronous increase of [Ca²⁺] across the cell.⁴

The ability of I_Ca to trigger Ca²⁺ release from the SR is reduced in myocytes from failing rat hearts.⁹⁵ It has been suggested that this may be due to changes in the colocalization of the L-type Ca²⁺ channel and RyR, increased separation of the t-tubule from the SR, or t-tubule remodeling. A confounding factor is that 4 different isoforms of the _1C subunit of the L-type Ca²⁺ channel are expressed in the normal human heart,¹⁰⁸ and isoform switching occurs in failing human myocytes. In addition, several lines of evidence indicate hyperphosphorylation of PKA target proteins in human heart failure,¹⁰² and because key target proteins appear to be localized at the t-tubules (above), changes in protein phosphorylation at the t-tubules may also underlie changes in excitation-contraction coupling.¹⁰⁴

However, many of these proposed mechanisms rely on a change in the efficiency with which I_Ca triggers SR Ca²⁺ release. It has been cogently argued that such a mechanism can produce only short-lived changes in the size of the Ca²⁺ transient and that it is more likely that a decrease in SR Ca²⁺ content underlies the decrease in the Ca²⁺ transient.¹⁰⁹ However, it remains possible that a reduction in t-tubule density in heart failure^5,6 could desynchronize Ca²⁺ release on electrical stimulation,^1,57 reducing peak [Ca²⁺]_i and slowing its time course.¹¹⁰

The location of NCX is also of interest because the activity of this protein appears to be upregulated in heart failure, which can improve contractile function^111,112 but may also lead to arrhythmias¹¹³; however, it is unclear whether it retains its t-tubular location during failure.

	Conclusions and Unanswered Questions

Top Abstract Introduction Occurrence and Morphology of... Localization of Function Functional Role Changes During Development and... Conclusions and Unanswered... References

 
It is clear that the t-tubules are not simple invaginations of the sarcolemma. Ca²⁺-handling proteins in particular appear to be located predominantly within the t-tubules, which therefore play a central role in excitation-contraction coupling, functioning as the major site for Ca²⁺ entry and release, allowing synchronous Ca²⁺ release throughout the cell and Ca²⁺ removal from the cytoplasm. The t-tubules may also be an important site for modulation of contraction via NCX. What is less clear is whether the density of these proteins is greater within the t-tubules than at the surface membrane, which has important implications for protein trafficking, or whether the fraction of these proteins within the t-tubules simply reflects the fraction of membrane within the t-tubules. Accurate estimates of protein and membrane fraction are required, although it is notable that despite the problems associated with immunohistochemical and electrophysiological investigation of the t-tubules, both techniques show apparent concentration of some proteins within the t-tubules, and many ultrastructural studies agree with capacitance measurements of the amount of cell membrane (30%) located in the t-tubules.

There are, however, many other questions that have not been fully answered: how are the t-tubules maintained, and how and why do they change during development, hypertrophy, and heart failure? How are the proteins concentrated at the t-tubules targeted to this part of the cell membrane? How does protein distribution change during development, hypertrophy, and failure, and what role does this play in the altered function observed in these conditions? What are the electrical properties of the t-tubules, and why are Ca²⁺-efflux pathways located close to Ca²⁺-release sites? These and many other questions will provide a future challenge in elucidating and understanding the role and the importance of the t-tubules in health and disease.

	Acknowledgments

 
The authors acknowledge financial support from the Wellcome Trust and British Heart Foundation and thank Dr Richard Sainson for helpful discussion and Tim Lee for the preparation of Figure 2.

	Footnotes

 
Original received March 3, 2003; revision received April 22, 2003; accepted April 22, 2003.

	References

Top Abstract Introduction Occurrence and Morphology of... Localization of Function Functional Role Changes During Development and... Conclusions and Unanswered... References

 
1. Heinzel FR, Bito V, Volders PG, Antoons G, Mubagwa K, Sipido KR. Spatial and temporal inhomogeneities during Ca²⁺ release from the sarcoplasmic reticulum in pig ventricular myocytes. Circ Res. 2002; 91: 1023–1030.[Abstract/Free Full Text]

2. Soeller C, Cannell MB. Examination of the transverse tubular system in living cardiac rat myocytes by 2-photon microscopy and digital image–processing techniques. Circ Res. 1999; 84: 266–275.[Abstract/Free Full Text]

3. Forbes MS, van Neil EE. Membrane systems of guinea pig myocardium: ultrastructure and morphometric studies. Anat Rec. 1988; 222: 362–379.[CrossRef][Medline] [Order article via Infotrieve]

4. Haddock PS, Coetzee WA, Cho E, Porter L, Katoh H, Bers DM, Jafri MS, Artman M. Subcellular [Ca²⁺]_i gradients during excitation-contraction coupling in newborn rabbit ventricular myocytes. Circ Res. 1999; 85: 415–427.[Abstract/Free Full Text]

5. He J, Conklin MW, Foell JD, Wolff MR, Haworth RA, Coronado R, Kamp TJ. Reduction in density of transverse tubules and L-type Ca²⁺ channels in canine tachycardia-induced heart failure. Cardiovasc Res. 2001; 49: 298–307.[Abstract/Free Full Text]

6. Wong C, Soeller C, Burton L, Cannell MB. Changes in transverse-tubular system architecture in myocytes from diseased human ventricles. Biophys J. 2002; 82: a588. Abstract.

7. Bossen EH, Sommer JR, Waugh RA. Comparative stereology of the mouse and finch left ventricle. Tissue Cell. 1978; 10: 773–784.[CrossRef][Medline] [Order article via Infotrieve]

8. Bossen EH, Sommer JR. Comparative stereology of the lizard and frog myocardium. Tissue Cell. 1984; 16: 173–178.[CrossRef][Medline] [Order article via Infotrieve]

9. Ayettey AS, Navaratnam V. The T-tubule system in the specialized and general myocardium of the rat. J Anat. 1978; 127: 125–140.[Medline] [Order article via Infotrieve]

10. Kirk MM, Izu LT, Chen-Izu Y, McCulle SL, Wier WG, Balke CW, Shorofsky SR. Role of the transverse-axial tubule system in generating calcium sparks and calcium transients in rat atrial myocytes. J Physiol. 2003; 547: 441–451.[Abstract/Free Full Text]

11. Bers DM. Excitation-Contraction Coupling and Cardiac Contractile Force. 2nd ed. Dordrecht, Netherlands: Kluwer Academic Publishers; 2001.

12. Page E. Quantitative ultrastructural analysis in cardiac membrane physiology. Am J Physiol. 1978; 235: C147–C158.[Medline] [Order article via Infotrieve]

13. Sommer JR, Jennings RB. Ultrastructure of Cardiac Muscle. New York, NY: Raven Press; 1992.

14. Forbes MS, Hawkey LA, Sperelakis N. The transverse-axial tubular system (TATS) of mouse myocardium: its morphology in the developing and adult animal. Am J Anat. 1984; 170: 143–162.[CrossRef][Medline] [Order article via Infotrieve]

15. Page E, McCallister LP, Power B. Sterological measurements of cardiac ultrastructures implicated in excitation-contraction coupling. Proc Natl Acad Sci U S A. 1971; 68: 1465–1466.[Abstract/Free Full Text]

16. Blatter LA, Niggli E. Confocal near-membrane detection of calcium in cardiac myocytes. Cell Calcium. 1998; 23: 269–279.[CrossRef][Medline] [Order article via Infotrieve]

17. Yao A, Spitzer KW, Ito N, Zaniboni M, Lorell BH, Barry WH. The restriction of diffusion of cations at the external surface of cardiac myocytes varies between species. Cell Calcium. 1997; 22: 431–438.[CrossRef][Medline] [Order article via Infotrieve]

18. Chen F, Mottino G, Klitzner TS, Philipson KD, Frank JS. Distribution of the Na⁺/Ca²⁺ exchange protein in developing rabbit myocytes. Am J Physiol. 1995; 268: C1126–C1132.[Medline] [Order article via Infotrieve]

19. Mitcheson JS, Hancox JC, Levi AJ. Action potentials, ion channel currents and transverse tubule density in adult rabbit ventricular myocytes maintained for 6 days in cell culture. Pflugers Arch. 1996; 431: 814–827.[Medline] [Order article via Infotrieve]

20. Kostin S, Scholz D, Shimada T, Maeno Y, Mollnau H, Hein S, Schaper J. The internal and external protein scaffold of the T-tubular system in cardiomyocytes. Cell Tissue Res. 1998; 294: 449–460.[CrossRef][Medline] [Order article via Infotrieve]

21. Brandt N. Identification of two populations of cardiac microsomes with nitrendipine receptors: correlation of the distribution of dihydropyridine receptors with organelle specific markers. Arch Biochem Biophys. 1985; 242: 306–319.[CrossRef][Medline] [Order article via Infotrieve]

22. Takagishi Y, Yasui K, Severs NJ, Murata Y. Species-specific difference in distribution of voltage-gated L-type Ca²⁺ channels of cardiac myocytes. Am J Physiol. 2000; 279: C1963–C1969.

23. Carl SL, Felix K, Caswell AH, Brandt NR, Ball WJ Jr, Vaghy PL, Meissner G, Ferguson DG. Immunolocalization of sarcolemmal dihydropyridine receptor and sarcoplasmic reticular triadin and ryanodine receptor in rabbit ventricle and atrium. J Cell Biol. 1995; 129: 672–682.[Medline] [Order article via Infotrieve]

24. Kawai M, Hussain M, Orchard CH. Excitation-contraction coupling in rat ventricular myocytes after formamide-induced detubulation. Am J Physiol. 1999; 277: H603–H609.[Medline] [Order article via Infotrieve]

25. Musa H, Lei M, Honjo H, Jones SA, Dobrzynski H, Lancaster MK, Takagishi Y, Henderson Z, Kodama I, Boyett MR. Heterogeneous expression of Ca²⁺ handling proteins in rabbit sinoatrial node. J Histochem Cytochem. 2002; 50: 311–324.[Abstract/Free Full Text]

26. Scriven DR, Dan P, Moore ED. Distribution of proteins implicated in excitation-contraction coupling in rat ventricular myocytes. Biophys J. 2000; 79: 2682–2691.[Medline] [Order article via Infotrieve]

27. Doyle DD, Kamp TJ, Palfrey HC, Miller RJ, Page E. Separation of cardiac plasmalemma into cell surface and T-tubular components: distribution of saxitoxin- and nitrendipine-binding sites. J Biol Chem. 1986; 261: 6556–6563.[Abstract/Free Full Text]

28. Frank JS, Mottino G, Reid D, Molday RS, Philipson KD. Distribution of the Na⁺-Ca²⁺ exchange protein in mammalian cardiac myocytes: an immunofluorescence and immunocolloidal gold-labeling study. J Cell Biol. 1992; 117: 337–345.[Abstract/Free Full Text]

29. Kieval RS, Bloch RJ, Lindenmayer GE, Ambesi A, Lederer WJ. Immunofluorescence localization of the Na-Ca exchanger in heart cells. Am J Physiol. 1992; 263: C545–C550.[Medline] [Order article via Infotrieve]

30. Petrecca K, Atanasiu R, Grinstein S, Orlowski J, Shrier A. Subcellular localization of the Na⁺/H⁺ exchanger NHE1 in rat myocardium. Am J Physiol. 1999; 276: H709–H717.[Medline] [Order article via Infotrieve]

31. McDonough AA, Zhang Y, Shin V, Frank JS. Subcellular distribution of sodium pump isoform subunits in mammalian cardiac myocytes. Am J Physiol. 1996; 270: C1221–C1227.[Medline] [Order article via Infotrieve]

32. Maier SKG, Westenbroek RE, Schenkman KA, Feigl EO, Scheuer T, Catterall WA. An unexpected role for brain-type sodium channels in coupling of cell surface depolarization to contraction in the heart. Proc Natl Acad Sci U S A. 2002; 99: 4073–4078.[Abstract/Free Full Text]

33. Barry DM, Trimmer JS, Merlie JP, Nerbonne JM. Differential expression of voltage-gated K⁺ channel subunits in adult rat heart: relation to functional K⁺ channels? Circ Res. 1995; 77: 361–369.[Abstract/Free Full Text]

34. Takeuchi S, Takagishi Y, Yasui K, Murata Y, Toyama J, Kodama I. Voltage-gated K⁺ channel, Kv4.2, localizes predominantly to the transverse-axial tubular system of the rat myocyte. J Mol Cell Cardiol. 2000; 32: 1361–1369.[CrossRef][Medline] [Order article via Infotrieve]

35. Jones SA, Morton MJ, Hunter M, Boyett MR. Expression of TASK-1, a pH-sensitive twin-pore domain K⁺ channel, in rat myocytes. Am J Physiol. 2002; 283: H181–H185.

36. Clark RB, Tremblay A, Melnyk P, Allen BG, Giles WR, Fiset C. T-tubule localization of the inward-rectifier K⁺ channel in mouse ventricular myocytes: a role in K⁺ accumulation. J Physiol. 2001; 537: 979–992.[Abstract/Free Full Text]

37. Pucéat M, Korichneva I, Cassoly R, Vassort G. Identification of band 3-like proteins and Cl^-/HCO₃^- exchange in isolated cardiomyocytes. J Biol Chem. 1995; 270: 1315–1322.[Abstract/Free Full Text]

38. Jurevicius J, Fischmeister R. cAMP compartmentation is responsible for a local activation of cardiac Ca²⁺ channels by ß-adrenergic agonists. Proc Natl Acad Sci U S A. 1996; 93: 295–299.[Abstract/Free Full Text]

39. Gao T, Puri TS, Gerhardstein BL, Chien AJ, Green RD, Hosey MM. Identification and subcellular localization of the subunits of L-type calcium channels and adenylyl cyclase in cardiac myocytes. J Biol Chem. 1997; 272: 19401–19407.[Abstract/Free Full Text]

40. Laflamme MA, Becker PL. G_s and adenylyl cyclase in transverse tubules of heart: implications for cAMP-dependent signaling. Am J Physiol. 1999; 277: H1841–H1848.[Medline] [Order article via Infotrieve]

41. Yang J, Drazba JA, Ferguson DG, Bond M. A-kinase anchoring protein 100 (AKAP100) is localized in multiple subcellular compartments in the adult rat heart. J Cell Biol. 1998; 142: 511–522.[Abstract/Free Full Text]

42. Santana LF, Chase EG, Votaw VS, Nelson MT, Greven R. Functional coupling of calcineurin and protein kinase A in mouse ventricular myocytes. J Physiol. 2002; 544: 57–69.[Abstract/Free Full Text]

43. Davare MA, Avdonin V, Hall DD, Peden EM, Burette A, Weinberg RJ, Horne MC, Hoshi T, Hell JW. A ß2 adrenergic receptor signaling complex assembled with the Ca²⁺ channel Cav1.2. Science. 2001; 293: 98–101.[Abstract/Free Full Text]

44. Barouch LA, Harrison RW, Skaf MW, Rosas GO, Cappola TP, Kobeissi ZA, Hobai IA, Lemmon CA, Burnett AL, O’Rourke B, Rodriguez ER, Huang PL, Lima JA, Berkowitz DE, Hare JM. Nitric oxide regulates the heart by spatial confinement of nitric oxide synthase isoforms. Nature. 2002; 416: 337–339.[Medline] [Order article via Infotrieve]

45. Blaustein MP, Lederer WJ. Sodium/calcium exchange: its physiological implications. Physiol Rev. 1999; 79: 763–854.[Abstract/Free Full Text]

46. Lipp P, Bootman MD. The physiology and molecular biology of cardiac Na/Ca exchange. In: Zipes DP, Jalife J, eds. Cardiac Electrophysiology From Cell to Bedside. Philadelphia, Pa: WB Saunders Co; 2000.

47. Wier WG, Balke CW. Ca²⁺ release mechanisms, Ca²⁺ sparks, and local control of excitation-contraction coupling in normal heart muscle. Circ Res. 1999; 85: 770–776.[Free Full Text]

48. Franzini-Armstrong C, Protasi F, Ramesh V. Comparative ultrastructure of Ca²⁺ release units in skeletal and cardiac muscle. Ann N Y Acad Sci. 1998; 853: 20–30.[CrossRef][Medline] [Order article via Infotrieve]

49. Jorgensen AO, Shen AC, Daly P, MacLennan DH. Localization of Ca²⁺+Mg²⁺-ATPase of the sarcoplasmic reticulum in adult rat papillary muscle. J Cell Biol. 1982; 93: 883–892.[Abstract/Free Full Text]

50. Evans AM, Cannell MB. The role of L-type Ca²⁺ current and Na⁺ current-stimulated Na/Ca exchange in triggering SR calcium release in guinea-pig cardiac ventricular myocytes. Cardiovasc Res. 1997; 35: 294–302.[Abstract/Free Full Text]

51. Sipido KR, Maes M, Van de WF. Low efficiency of Ca²⁺ entry through the Na⁺-Ca²⁺ exchanger as trigger for Ca²⁺ release from the sarcoplasmic reticulum: a comparison between L-type Ca²⁺ current and reverse-mode Na⁺-Ca²⁺ exchange. Circ Res. 1997; 81: 1034–1044.[Abstract/Free Full Text]

52. Leblanc N, Hume JR. Sodium current-induced release of calcium from cardiac sarcoplasmic reticulum. Science. 1990; 248: 372–376.[Abstract/Free Full Text]

53. Trafford AW, Diaz ME, O’Neill SC, Eisner DA. Comparison of subsarcolemmal and bulk calcium concentration during spontaneous calcium release in rat ventricular myocytes. J Physiol. 1995; 488: 577–586.[Abstract/Free Full Text]

54. Cordeiro JM, Spitzer KW, Giles WR, Ershler PE, Cannell MB, Bridge JH. Location of the initiation site of calcium transients and sparks in rabbit heart Purkinje cells. J Physiol. 2001; 531: 301–314.[Abstract/Free Full Text]

55. Huser J, Lipsius SL, Blatter LA. Calcium gradients during excitation-contraction coupling in cat atrial myocytes. J Physiol. 1996; 494: 641–651.[Abstract/Free Full Text]

56. Lipp P, Huser J, Pott L, Niggli E. Spatially non-uniform Ca²⁺ signals induced by the reduction of transverse tubules in citrate-loaded guinea-pig ventricular myocytes in culture. J Physiol. 1996; 497: 589–597.[Abstract/Free Full Text]

57. Brette F, Komukai K, Orchard CH. Validation of formamide as a detubulation agent in isolated rat cardiac cells. Am J Physiol. 2002; 283: H1720–H1728.

58. Yang Z, Pascarel C, Steele DS, Komukai K, Brette F, Orchard CH. Na⁺-Ca²⁺ exchange activity is localized in the t-tubules of rat ventricular myocytes. Circ Res. 2002; 91: 315–322.[Abstract/Free Full Text]

59. Shacklock PS, Wier WG, Balke CW. Local Ca²⁺ transients (Ca²⁺ sparks) originate at transverse tubules in rat heart cells. J Physiol. 1995; 487: 601–608.[Abstract/Free Full Text]

60. Christé G. Localization of K⁺ channels in the T-tubules of cardiomyocytes as suggested by the parallel decay of membrane capacitance, IK₁ and IK_ATP during culture and by delayed IK₁ response to barium. J Mol Cell Cardiol. 1999; 31: 2207–2213.[CrossRef][Medline] [Order article via Infotrieve]

61. Shepherd N, McDonough HB. Ionic diffusion in transverse tubules of cardiac ventricular myocytes. Am J Physiol. 1998; 275: H852–H860.[Medline] [Order article via Infotrieve]

62. Korchev YE, Negulyaev YA, Edwards CR, Vodyanoy I, Lab MJ. Functional localization of single active ion channels on the surface of a living cell. Nat Cell Biol. 2000; 2: 616–619.[CrossRef][Medline] [Order article via Infotrieve]

63. Komukai K, Brette F, Yamanushi TT, Orchard CH. K⁺ current distribution in rat sub-epicardial ventricular myocytes. Pflugers Arch. 2002; 444: 532–538.[CrossRef][Medline] [Order article via Infotrieve]

64. Brette F, Komukai K, Orchard CH. Importance of T-tubules in the response of cardiac myocytes to ß-adrenergic stimulation. Biophys J. 2002; 82: a96. Abstract.

65. Huxley AF, Taylor RE. Local activation of striated muscle fibres. J Physiol. 1958; 144: 426–441.[Free Full Text]

66. Katz MA. Physiology of the Heart. New York, NY: Raven Press; 1992.

67. Tidball JG, Smith R, Shattock MJ, Bers DM. Differences in action potential configuration in ventricular trabeculae correlate with differences in density of transverse tubule-sarcoplasmic reticulum couplings. J Mol Cell Cardiol. 1988; 20: 539–546.[CrossRef][Medline] [Order article via Infotrieve]

68. Jack JJB, Noble D, Tsien RW. Electric Current Flow in Excitable Cells. Oxford, UK: Oxford University Press; 1985.

69. Kim AM, Vergara JL. Supercharging accelerates T-tubule membrane potential changes in voltage clamped frog skeletal muscle fibers. Biophys J. 1998; 75: 2098–2116.[Medline] [Order article via Infotrieve]

70. Fabiato A. Calcium-induced release of calcium from the cardiac sarcoplasmic reticulum. Am J Physiol. 1983; 245: C1–C14.[Medline] [Order article via Infotrieve]

71. Goldhaber JI, Lamp ST, Walter DO, Garfinkel A, Fukumoto GH, Weiss JN. Local regulation of the threshold for calcium sparks in rat ventricular myocytes: role of sodium-calcium exchange. J Physiol. 1999; 520: 431–438.[Abstract/Free Full Text]

72. Cheng H, Lederer WJ, Cannell MB. Calcium sparks: elementary events underlying excitation-contraction coupling in heart muscle. Science. 1993; 262: 740–744.[Abstract/Free Full Text]

73. Tanaka H, Sekine T, Kawanishi T, Nakamura R, Shigenobu K. Intrasarcomere [Ca²⁺] gradients and their spatio-temporal relation to Ca²⁺ sparks in rat cardiomyocytes. J Physiol. 1998; 508: 145–152.[Abstract/Free Full Text]

74. Song LS, Sham JS, Stern MD, Lakatta EG, Cheng H. Direct measurement of SR release flux by tracking "Ca²⁺ spikes" in rat cardiac myocytes. J Physiol. 1998; 512: 677–691.[Abstract/Free Full Text]

75. Blatter LA, Kockskamper J, Sheehan KA, Zima AV, Huser J, Lipsius SL. Local calcium gradients during excitation-contraction coupling and alternans in atrial myocytes. J Physiol. 2003; 546: 19–31.[Abstract/Free Full Text]

76. Brette F, Orchard CH. Effect of ß-adrenergic stimulation on subcellular Ca gradients in control and detubulated rat ventricular myocytes. Biophys J. 2003; 84: a200. Abstract.

77. Song LS, Wang SQ, Xiao RP, Spurgeon H, Lakatta EG, Cheng H. ß-Adrenergic stimulation synchronizes intracellular Ca²⁺ release during excitation-contraction coupling in cardiac myocytes. Circ Res. 2001; 88: 794–801.[Abstract/Free Full Text]

78. Levi AJ, Spitzer KW, Kohmoto O, Bridge JH. Depolarization-induced Ca entry via Na-Ca exchange triggers SR release in guinea pig cardiac myocytes. Am J Physiol. 1994; 266: H1422–H1433.[Medline] [Order article via Infotrieve]

79. Lederer WJ, Niggli E, Hadley RW. Sodium-calcium exchange in excitable cells: fuzzy space. Science. 1990; 248: 283.[Free Full Text]

80. Janvier NC, Boyett MR. The role of Na-Ca exchange current in the cardiac action potential. Cardiovasc Res. 1996; 32: 69–84.[CrossRef][Medline] [Order article via Infotrieve]

81. Zaccolo M, Pozzan T. Discrete microdomains with high concentration of cAMP in stimulated rat neonatal cardiac myocytes. Science. 2002; 295: 1711–1715.[Abstract/Free Full Text]

82. Xiao RP. ß-Adrenergic signaling in the heart: dual coupling of the ß2-adrenergic receptor to G_s and G_i proteins. Sci STKE. 2001; 2001: RE15.[Medline] [Order article via Infotrieve]

83. Brette F, Rodriguez P, Coyler J, Orchard CH. Effect of detubulation on phospholamban phosphorylation in rat ventricular myocytes. J Physiol. 2002; 544P: 46P. Abstract.

84. Smolich JJ. Ultrastructural and functional features of the developing mammalian heart: a brief overview. Reprod Fertil Dev. 1995; 7: 451–461.[CrossRef][Medline] [Order article via Infotrieve]

85. Lee E, Marcucci M, Daniell L, Pypaert M, Weisz OA, Ochoa GC, Farsad K, Wenk MR, De Camilli P. Amphiphysin 2 (Bin1) and T-tubule biogenesis in muscle. Science. 2002; 297: 1193–1196.[Abstract/Free Full Text]

86. Carozzi AJ, Ikonen E, Lindsay MR, Parton RG. Role of cholesterol in developing T-tubules: analogous mechanisms for T-tubule and caveolae biogenesis. Traffic. 2000; 1: 326–341.[CrossRef][Medline] [Order article via Infotrieve]

87. Razani B, Woodman SE, Lisanti MP. Caveolae: from cell biology to animal physiology. Pharmacol Rev. 2002; 54: 431–467.[Abstract/Free Full Text]

88. Franzini-Armstrong C. Simultaneous maturation of transverse tubules and sarcoplasmic reticulum during muscle differentiation in the mouse. Dev Biol. 1991; 146: 353–363.[CrossRef][Medline] [Order article via Infotrieve]

89. Ishikawa H. Formation of elaborate networks of T-system tubules in cultured skeletal muscle with special reference to the T-system formation. J Cell Biol. 1968; 38: 51–66.[Abstract/Free Full Text]

90. Page E, McCallister LP. Quantitative electron microscopic description of heart muscle cells: application to normal, hypertrophied and thyroxin-stimulated hearts. Am J Cardiol. 1973; 31: 172–181.[CrossRef][Medline] [Order article via Infotrieve]

91. Keung EC, Toll L, Ellis M, Jensen RA. L-type cardiac calcium channels in doxorubicin cardiomyopathy in rats morphological, biochemical, and functional correlations. J Clin Invest. 1991; 87: 2108–2113.[Medline] [Order article via Infotrieve]

92. Schaper J, Froede R, Hein S, Buck A, Hashizume H, Speiser B, Friedl A, Bleese N. Impairment of the myocardial ultrastructure and changes of the cytoskeleton in dilated cardiomyopathy. Circulation. 1991; 83: 504–514.[Abstract/Free Full Text]

93. Kaprielian RR, Stevenson S, Rothery SM, Cullen MJ, Severs NJ. Distinct patterns of dystrophin organization in myocyte sarcolemma and transverse tubules of normal and diseased human myocardium. Circulation. 2000; 101: 2586–2594.[Abstract/Free Full Text]

94. Ohler A, Houser SR, Tomaselli GF, Rourke B. Transverse tubules are unchanged in myocytes from failing human heart. Biophys J. 2002; 82: a590. Abstract.

95. Gomez AM, Valdivia HH, Cheng H, Lederer MR, Santana LF, Cannell MB, McCune SA, Altschuld RA, Lederer WJ. Defective excitation-contraction coupling in experimental cardiac hypertrophy and heart failure. Science. 1997; 276: 800–806.[Abstract/Free Full Text]

96. Gomez AM, Guatimosim S, Dilly KW, Vassort G, Lederer WJ. Heart failure after myocardial infarction: altered excitation-contraction coupling. Circulation. 2001; 104: 688–693.[Abstract/Free Full Text]

97. Page E, Buecker JL. Development of dyadic junctional complexes between sarcoplasmic reticulum and plasmalemma in rabbit left ventricular myocardial cells: morphometric analysis. Circ Res. 1981; 48: 519–522.[Abstract/Free Full Text]

98. Cohen NM, Lederer WJ. Changes in the calcium current of rat heart ventricular myocytes during development. J Physiol. 1988; 406: 115–146.[Abstract/Free Full Text]

99. Haddock PS, Coetzee WA, Artman M. Na⁺/Ca²⁺ exchange current and contractions measured under Cl^--free conditions in developing rabbit hearts. Am J Physiol. 1997; 273: H837–H846.[Medline] [Order article via Infotrieve]

100. Bers DM, Philipson KD, Langer GA. Cardiac contractility and sarcolemmal calcium binding in several cardiac muscle preparations. Am J Physiol. 1981; 240: H576–H583.[Medline] [Order article via Infotrieve]

101. Tomaselli GF, Marban E. Electrophysiological remodeling in hypertrophy and heart failure. Cardiovasc Res. 1999; 42: 270–283.[Free Full Text]

102. Marx SO, Reiken S, Hisamatsu Y, Jayaraman T, Burkhoff D, Rosemblit N, Marks AR. PKA phosphorylation dissociates FKBP12.6 from the calcium release channel (ryanodine receptor): defective regulation in failing hearts. Cell. 2000; 101: 365–376.[CrossRef][Medline] [Order article via Infotrieve]

103. Benitah JP, Gomez AM, Fauconnier J, Kerfant BG, Perrier E, Vassort G, Richard S. Voltage-gated Ca²⁺ currents in the human pathophysiologic heart: a review. Basic Res Cardiol. 2002; 97 (suppl 1): I-11–I-18.[Medline] [Order article via Infotrieve]

104. Chen X, Piacentino V III, Furukawa S, Goldman B, Margulies KB, Houser SR. L-type Ca²⁺ channel density and regulation are altered in failing human ventricular myocytes and recover after support with mechanical assist devices. Circ Res. 2002; 91: 517–524.[Abstract/Free Full Text]

105. Schroder F, Handrock R, Beuckelmann DJ, Hirt S, Hullin R, Priebe L, Schwinger RHG, Weil J, Herzig S. Increased availability and open probability of single L-type calcium channels from failing compared with nonfailing human ventricle. Circulation. 1998; 98: 969–976.[Abstract/Free Full Text]

106. Barry WH. Na⁺-Ca²⁺ exchange in failing myocardium: friend or foe? Circ Res. 2000; 87: 529–531.[Free Full Text]

107. Gomez AM, Schwaller B, Porzig H, Vassort G, Niggli E, Egger M. Increased exchange current but normal Ca²⁺ transport via Na⁺-Ca²⁺ exchange during cardiac hypertrophy after myocardial infarction. Circ Res. 2002; 91: 323–330.[Abstract/Free Full Text]

108. Yang Y, Chen X, Margulies K, Jeevanandam V, Pollack P, Bailey BA, Houser SR. L-type Ca²⁺ channel 1c subunit isoform switching in failing human ventricular myocardium. J Mol Cell Cardiol. 2000; 32: 973–984.[CrossRef][Medline] [Order article via Infotrieve]

109. Eisner DA, Trafford AW. Heart failure and the ryanodine receptor: does Occam’s razor rule? Circ Res. 2002; 91: 979–981.[Free Full Text]

110. Litwin SE, Zhang D, Bridge JH. Dyssynchronous Ca²⁺ sparks in myocytes from infarcted hearts. Circ Res. 2000; 87: 1040–1047.[Abstract/Free Full Text]

111. Mattiello JA, Margulies KB, Jeevanandam V, Houser SR. Contribution of reverse-mode sodium-calcium exchange to contractions in failing human left ventricular myocytes. Cardiovasc Res. 1998; 37: 424–431.[Abstract/Free Full Text]

112. Sipido KR, Volders PG, De Groot SH, Verdonck F, Van de Werf F, Wellens HJ, Vos MA. Enhanced Ca²⁺ release and Na/Ca exchange activity in hypertrophied canine ventricular myocytes: potential link between contractile adaptation and arrhythmogenesis. Circulation. 2000; 102: 2137–2144.[Abstract/Free Full Text]

113. Pogwizd SM, Schlotthauer K, Li L, Yuan W, Bers DM. Arrhythmogenesis and contractile dysfunction in heart failure: roles of sodium-calcium exchange, inward rectifier potassium current, and residual ß-adrenergic responsiveness. Circ Res. 2001; 88: 1159–1167.[Abstract/Free Full Text]

This article has been cited by other articles:

				I. Lenaerts, V. Bito, F. R. Heinzel, R. B. Driesen, P. Holemans, J. D'hooge, H. Heidbuchel, K. R. Sipido, and R. Willems Ultrastructural and Functional Remodeling of the Coupling Between Ca2+ Influx and Sarcoplasmic Reticulum Ca2+ Release in Right Atrial Myocytes From Experimental Persistent Atrial Fibrillation Circ. Res., October 23, 2009; 105(9): 876 - 885. [Abstract] [Full Text] [PDF]

				K. M. Dibb, J. D. Clarke, M. A. Horn, M. A. Richards, H. K. Graham, D. A. Eisner, and A. W. Trafford Characterization of an Extensive Transverse Tubular Network in Sheep Atrial Myocytes and its Depletion in Heart Failure Circ Heart Fail, September 1, 2009; 2(5): 482 - 489. [Abstract] [Full Text] [PDF]

				J. Liu, D. K. Lieu, C. W. Siu, J.-D. Fu, H.-F. Tse, and R. A. Li Facilitated maturation of Ca2+ handling properties of human embryonic stem cell-derived cardiomyocytes by calsequestrin expression Am J Physiol Cell Physiol, July 1, 2009; 297(1): C152 - C159. [Abstract] [Full Text] [PDF]

				C. H. Orchard, M. Pasek, and F. Brette The role of mammalian cardiac t-tubules in excitation-contraction coupling: experimental and computational approaches Exp Physiol, May 1, 2009; 94(5): 509 - 519. [Abstract] [Full Text] [PDF]

				Abstracts Heart, February 1, 2009; 95(4): e1 - e1. [Full Text] [PDF]

				J. O-Uchi, H. Sasaki, S. Morimoto, Y. Kusakari, H. Shinji, T. Obata, K. Hongo, K. Komukai, and S. Kurihara Interaction of {alpha}1-Adrenoceptor Subtypes With Different G Proteins Induces Opposite Effects on Cardiac L-type Ca2+ Channel Circ. Res., June 6, 2008; 102(11): 1378 - 1388. [Abstract] [Full Text] [PDF]

				K. Y. Chung, M. Kang, and J. W. Walker Contractile regulation by overexpressed ETA requires intact T tubules in adult rat ventricular myocytes Am J Physiol Heart Circ Physiol, May 1, 2008; 294(5): H2391 - H2399. [Abstract] [Full Text] [PDF]

				J. N. Dominguez, A. de la Rosa, F. Navarro, D. Franco, and A. E. Aranega Tissue distribution and subcellular localization of the cardiac sodium channel during mouse heart development Cardiovasc Res, April 1, 2008; 78(1): 45 - 52. [Abstract] [Full Text] [PDF]

				T. Banyasz, I. Lozinskiy, C. E. Payne, S. Edelmann, B. Norton, B. Chen, Y. Chen-Izu, L. T. Izu, and C. W. Balke Transformation of adult rat cardiac myocytes in primary culture Exp Physiol, March 1, 2008; 93(3): 370 - 382. [Abstract] [Full Text] [PDF]

				F. R. Heinzel, V. Bito, L. Biesmans, M. Wu, E. Detre, F. von Wegner, P. Claus, S. Dymarkowski, F. Maes, J. Bogaert, et al. Remodeling of T-Tubules and Reduced Synchrony of Ca2+ Release in Myocytes From Chronically Ischemic Myocardium Circ. Res., February 15, 2008; 102(3): 338 - 346. [Abstract] [Full Text] [PDF]

				D. X. Tishkoff, K. A. Nibbelink, K. H. Holmberg, L. Dandu, and R. U. Simpson Functional Vitamin D Receptor (VDR) in the T-Tubules of Cardiac Myocytes: VDR Knockout Cardiomyocyte Contractility Endocrinology, February 1, 2008; 149(2): 558 - 564. [Abstract] [Full Text] [PDF]

				C. Orchard and F. Brette t-tubules and sarcoplasmic reticulum function in cardiac ventricular myocytes Cardiovasc Res, January 15, 2008; 77(2): 237 - 244. [Abstract] [Full Text] [PDF]

				V. Bito, F. R. Heinzel, L. Biesmans, G. Antoons, and K. R. Sipido Crosstalk between L-type Ca2+ channels and the sarcoplasmic reticulum: alterations during cardiac remodelling Cardiovasc Res, January 15, 2008; 77(2): 315 - 324. [Abstract] [Full Text] [PDF]

				K. M. S. O'Connell, J. D. Whitesell, and M. M. Tamkun Localization and mobility of the delayed-rectifer K+ channel Kv2.1 in adult cardiomyocytes Am J Physiol Heart Circ Physiol, January 1, 2008; 294(1): H229 - H237. [Abstract] [Full Text] [PDF]

				K. R. Sipido and S. P. Janssens How Old Is Your Heart? Circ. Res., August 17, 2007; 101(4): 323 - 325. [Full Text] [PDF]

				A. O. Verkerk, A. C.G. van Ginneken, T. A.B. van Veen, and H. L. Tan Effects of heart failure on brain-type Na+ channels in rabbit ventricular myocytes Europace, August 1, 2007; 9(8): 571 - 577. [Abstract] [Full Text] [PDF]

				F. Brette and C. Orchard Resurgence of Cardiac T-Tubule Research Physiology, June 1, 2007; 22(3): 167 - 173. [Abstract] [Full Text] [PDF]

				M. Kang, K. Y. Chung, and J. W. Walker G-Protein Coupled Receptor Signaling in Myocardium: Not for the Faint of Heart Physiology, June 1, 2007; 22(3): 174 - 184. [Abstract] [Full Text] [PDF]

				K. Y. Chung and J. W. Walker Interaction and Inhibitory Cross-Talk between Endothelin and ErbB Receptors in the Adult Heart Mol. Pharmacol., June 1, 2007; 71(6): 1494 - 1502. [Abstract] [Full Text] [PDF]

				H. E. D. J. ter Keurs and P. A. Boyden Calcium and Arrhythmogenesis Physiol Rev, April 1, 2007; 87(2): 457 - 506. [Abstract] [Full Text] [PDF]

				K. A. B. Davey, P. B. Garlick, A. Warley, and R. Southworth Immunogold labeling study of the distribution of GLUT-1 and GLUT-4 in cardiac tissue following stimulation by insulin or ischemia Am J Physiol Heart Circ Physiol, April 1, 2007; 292(4): H2009 - H2019. [Abstract] [Full Text] [PDF]

				L. Contreras, P. Gomez-Puertas, M. Iijima, K. Kobayashi, T. Saheki, and J. Satrustegui Ca2+ Activation Kinetics of the Two Aspartate-Glutamate Mitochondrial Carriers, Aralar and Citrin: ROLE IN THE HEART MALATE-ASPARTATE NADH SHUTTLE J. Biol. Chem., March 9, 2007; 282(10): 7098 - 7106. [Abstract] [Full Text] [PDF]

				M. Seeley, W. Huang, Z. Chen, W. O. Wolff, X. Lin, and X. Xu Depletion of Zebrafish Titin Reduces Cardiac Contractility by Disrupting the Assembly of Z-Discs and A-Bands Circ. Res., February 2, 2007; 100(2): 238 - 245. [Abstract] [Full Text] [PDF]

				V. Bito, J. van der Velden, P. Claus, C. Dommke, A. Van Lommel, L. Mortelmans, E. Verbeken, B. Bijnens, G. Stienen, and K. R. Sipido Reduced Force Generating Capacity in Myocytes From Chronically Ischemic, Hibernating Myocardium Circ. Res., February 2, 2007; 100(2): 229 - 237. [Abstract] [Full Text] [PDF]

				A. Segret, C. Rucker-Martin, C. Pavoine, J. Flavigny, E. Deroubaix, M.-A. Chatel, A. Lombet, and J.-F. Renaud Structural Localization and Expression of CXCL12 and CXCR4 in Rat Heart and Isolated Cardiac Myocytes J. Histochem. Cytochem., February 1, 2007; 55(2): 141 - 150. [Abstract] [Full Text] [PDF]

				J. Gorelik, L. Q. Yang, Y. Zhang, M. Lab, Y. Korchev, and S. E. Harding A novel Z-groove index characterizing myocardial surface structure Cardiovasc Res, December 1, 2006; 72(3): 422 - 429. [Abstract] [Full Text] [PDF]

				M. D. Bootman, D. R. Higazi, S. Coombes, and H. L. Roderick Calcium signalling during excitation-contraction coupling in mammalian atrial myocytes. J. Cell Sci., October 1, 2006; 119(Pt 19): 3915 - 3925. [Abstract] [Full Text] [PDF]

				F. Swift, T. A. Stromme, B. Amundsen, O. M. Sejersted, and I. Sjaastad Slow diffusion of K+ in the T tubules of rat cardiomyocytes J Appl Physiol, October 1, 2006; 101(4): 1170 - 1176. [Abstract] [Full Text] [PDF]

				C. Orchard T-tubule trouble J. Physiol., July 15, 2006; 574(2): 330 - 330. [Full Text] [PDF]

				M. Pasek, J. Simurda, and G. Christe The functional role of cardiac T-tubules explored in a model of rat ventricular myocytes Phil Trans R Soc A, May 15, 2006; 364(1842): 1187 - 1206. [Abstract] [Full Text] [PDF]

				F. Brette and C. H. Orchard No Apparent Requirement for Neuronal Sodium Channels in Excitation-Contraction Coupling in Rat Ventricular Myocytes Circ. Res., March 17, 2006; 98(5): 667 - 674. [Abstract] [Full Text] [PDF]

				S. Calaghan and E. White Caveolae modulate excitation-contraction coupling and {beta}2-adrenergic signalling in adult rat ventricular myocytes Cardiovasc Res, March 1, 2006; 69(4): 816 - 824. [Abstract] [Full Text] [PDF]

				B. P. Head, H. H. Patel, D. M. Roth, N. C. Lai, I. R. Niesman, M. G. Farquhar, and P. A. Insel G-protein-coupled Receptor Signaling Components Localize in Both Sarcolemmal and Intracellular Caveolin-3-associated Microdomains in Adult Cardiac Myocytes J. Biol. Chem., September 2, 2005; 280(35): 31036 - 31044. [Abstract] [Full Text] [PDF]

				L.S. Meadows and L.L. Isom Sodium channels as macromolecular complexes: Implications for inherited arrhythmia syndromes Cardiovasc Res, August 15, 2005; 67(3): 448 - 458. [Abstract] [Full Text] [PDF]

				J. O-Uchi, K. Komukai, Y. Kusakari, T. Obata, K. Hongo, H. Sasaki, and S. Kurihara {alpha}1-Adrenoceptor stimulation potentiates L-type Ca2+ current through Ca2+/calmodulin-dependent PK II (CaMKII) activation in rat ventricular myocytes PNAS, June 28, 2005; 102(26): 9400 - 9405. [Abstract] [Full Text] [PDF]

				H. A. Shiels and E. White Temporal and spatial properties of cellular Ca2+ flux in trout ventricular myocytes Am J Physiol Regulatory Integrative Comp Physiol, June 1, 2005; 288(6): R1756 - R1766. [Abstract] [Full Text] [PDF]

				B.-G. Kerfant, D. Gidrewicz, H. Sun, G. Y. Oudit, J. M. Penninger, and P. H. Backx Cardiac Sarcoplasmic Reticulum Calcium Release and Load Are Enhanced by Subcellular cAMP Elevations in PI3K{gamma}-Deficient Mice Circ. Res., May 27, 2005; 96(10): 1079 - 1086. [Abstract] [Full Text] [PDF]

				V. Haufe, J. A. Camacho, R. Dumaine, B. Gunther, C. Bollensdorff, G. S. von Banchet, K. Benndorf, and T. Zimmer Expression pattern of neuronal and skeletal muscle voltage-gated Na+ channels in the developing mouse heart J. Physiol., May 1, 2005; 564(3): 683 - 696. [Abstract] [Full Text] [PDF]

				C. G. Perez, J. A. Copello, Y. Li, K. L. Karko, L. Gomez, J. Ramos-Franco, M. Fill, A. L. Escobar, and R. Mejia-Alvarez Ryanodine receptor function in newborn rat heart Am J Physiol Heart Circ Physiol, May 1, 2005; 288(5): H2527 - H2540. [Abstract] [Full Text] [PDF]

				L. Mackenzie, H. L. Roderick, M. J. Berridge, S. J. Conway, and M. D. Bootman The spatial pattern of atrial cardiomyocyte calcium signalling modulates contraction J. Cell Sci., December 15, 2004; 117(26): 6327 - 6337. [Abstract] [Full Text] [PDF]

				E. M. C. Jones, E. C. Roti Roti, J. Wang, S. A. Delfosse, and G. A. Robertson Cardiac IKr Channels Minimally Comprise hERG 1a and 1b Subunits J. Biol. Chem., October 22, 2004; 279(43): 44690 - 44694. [Abstract] [Full Text] [PDF]

				V. E. Bondarenko, G. P. Szigeti, G. C. L. Bett, S.-J. Kim, and R. L. Rasmusson Computer model of action potential of mouse ventricular myocytes Am J Physiol Heart Circ Physiol, September 1, 2004; 287(3): H1378 - H1403. [Abstract] [Full Text] [PDF]

				F. Brette, L. Salle, and C. H. Orchard Differential Modulation of L-type Ca2+ Current by SR Ca2+ Release at the T-Tubules and Surface Membrane of Rat Ventricular Myocytes Circ. Res., July 9, 2004; 95(1): e1 - e7. [Abstract] [Full Text] [PDF]

				S. Hatem Does the loss of transverse tubules contribute to dyssynchronous Ca2+ release during heart failure? Cardiovasc Res, April 1, 2004; 62(1): 1 - 3. [Full Text] [PDF]

This Article Abstract Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Request Permissions Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Brette, F. Articles by Orchard, C. Search for Related Content PubMed PubMed Citation Articles by Brette, F. Articles by Orchard, C. Related Collections Pathophysiology Calcium cycling/excitation-contraction coupling Ion channels/membrane transport