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 McDonald, K. S. Articles by Moss, R. L. Search for Related Content PubMed PubMed Citation Articles by McDonald, K. S. Articles by Moss, R. L. Related Collections Structure Contractile function Calcium cycling/excitation-contraction coupling

(Circulation Research. 2000;87:768.)
© 2000 American Heart Association, Inc.

Cellular Biology

Strongly Binding Myosin Crossbridges Regulate Loaded Shortening and Power Output in Cardiac Myocytes

Kerry S. McDonald, Richard L. Moss

From the Department of Physiology (K.S.M.), University of Missouri School of Medicine, Columbia, Mo, and Department of Physiology (R.L.M.), University of Wisconsin Medical School, Madison, Wis.

Correspondence to Kerry S. McDonald, PhD, Department of Physiology, School of Medicine, University of Missouri, MA415 Medical Sciences Building, Columbia, MO 65212. E-mail mcdonaldks{at}missouri.edu \ © 2000 American Heart Association, Inc.

	Abstract

Top Abstract Introduction Materials and Methods Results Discussion References

 
Abstract—This study investigated the possible roles of strongly binding myosin crossbridges in determining loaded shortening and power output in cardiac myocytes. Single skinned cardiac myocytes were attached between a force transducer and position motor, and shortening velocities were measured over a range of loads during varying levels of Ca²⁺ activation. Lowering the [Ca²⁺] slowed shortening velocities, decreased relative power output, and increased the curvature of length traces. We tested the hypothesis that Ca²⁺ activation dependence of loaded shortening is determined primarily by strongly binding crossbridges or by [Ca²⁺] per se, which was done by measuring loaded shortening before and after addition of N-ethylmaleimide–conjugated myosin subfragment-1 (NEM-S1), a strongly binding myosin analogue that cooperatively enhances thin filament activation. At fixed [Ca²⁺], NEM-S1 reduced the curvature of length traces and sped loaded shortening velocities. Even when [Ca²⁺] was adjusted so that force was equal with and without NEM-S1, myocyte shortening was faster and exhibited less curvature with NEM-S1. In the presence of NEM-S1, peak relative power output was also significantly greater during activations either at the same [Ca²⁺] or when [Ca²⁺] was adjusted to achieve the same force. Consequently, NEM-S1 eliminated any Ca²⁺ dependence of relative power output that is normally observed in cardiac myocytes. These results indicate that strongly binding crossbridges play a significant role in determining loaded shortening and power output and suggest that previously observed Ca²⁺ dependence of power output is mediated by alterations in numbers of crossbridges bound to the thin filament.

Key Words: cardiac myocytes • power output • myosin

	Introduction

Top Abstract Introduction Materials and Methods Results Discussion References

 
Myocardium shortens against an afterload as blood is expelled into the circulation during systole. The amount of blood expelled is ultimately determined by the rate of myocardial shortening, which depends on several factors and varies inversely with afterload. Considerable information regarding the relationship between stroke volume and afterload has been obtained by characterizing force-velocity relationships in isolated, multicellular preparations of myocardium. Although these experiments have enriched our understanding of myocardial contraction, the presence of extracellular elasticity has confounded the interpretation of these relationships and made it difficult to assess how afterload influences the contractile machinery. The recent development of single skinned myocyte preparations for dynamic mechanical measurements circumvents several of the complexities associated with intercellular elasticity in multicellular preparations and has added to our understanding of myofibrillar behavior during loaded shortening. In single skinned myocyte preparations, as with multicellular preparations, force-velocity relationships are hyperbolic and are highly sensitive to changes in myoplasmic [Ca²⁺].^{1 2} Also, the time course of loaded shortening changes when Ca²⁺ activation levels are reduced in skinned myocyte preparations. Lowering the [Ca²⁺] not only yields slower shortening velocities but also increases the curvature in length traces during load clamps.^{1 3} Velocity slows progressively throughout isotonic shortening, which may have an important physiological role in determining hemodynamics during the latter part of systole. The purpose of this study was to gain a better understanding of the basis for this Ca²⁺-dependent slowing of myocyte velocity during shortening and, thus, decreased power output both at the onset of loaded shortening and throughout the isotonic contraction.

Myocardial activation and contraction involve complex interactions among Ca²⁺, thin filament regulatory proteins, and myosin crossbridges.⁴ Some current models of regulation propose that thin filaments exist in 3 distinct states, which depend on the relative position of tropomyosin.^{5 6} In the absence of Ca²⁺, thin filaments are thought to be in a "blocked" state in which tropomyosin sterically hinders myosin from interacting with actin. When Ca²⁺ binds to troponin C, the thin filament undergoes a transition to a "closed" state as tropomyosin changes position, which allows increased numbers of weakly bound actomyosin crossbridges to interact with actin. It has recently been proposed that when thin filaments are in the closed state a population of crossbridges undergoes the transition to a strongly bound, non–force-generating state,⁷ which promotes further movement of tropomyosin into an "open" state. It is only in the open state that strongly bound force-generating crossbridges can form between myosin and actin. Thus, in this model, full activation of thin filaments occurs only in the presence of both Ca²⁺ and strongly bound myosin crossbridges, wherein Ca²⁺ first allows crossbridges to bind and these in turn promote additional binding.

One well-established consequence of muscle shortening is a reduction in the number of strongly bound crossbridges,^{8 9} which, according to the above model, would tend to shift thin filaments from the open state to the "closed" state in a cooperative manner.^{10 11} In the context of loaded shortening, such a reduction in strongly bound force-generating crossbridges would result in slower crossbridge cycling rates for the muscle to sustain a given load. The resulting cooperative inactivation of the thin filament could mediate the characteristic slowing of muscle shortening under load in submaximally Ca²⁺-activated myocardium.¹² If such a mechanism is involved, we predict that experimental manipulations designed to maintain the open state or activated thin filaments during loaded shortening would attenuate curvilinear shortening. We undertook such experiments by examining the characteristics of loaded shortening in the presence of a strongly bound crossbridge derivative, N-ethylmaleimide–conjugated myosin subfragment-1 (NEM-S1), which is thought to cooperatively enhance thin filament activation. NEM-S1 activates acto-S1 ATPase activity in solution¹³ and both force and kinetics of force development in skinned muscle preparations.^{14 15} Thus, our experiments tested the hypothesis that curvilinear shortening in submaximally activated myocardium is modulated primarily by strongly binding crossbridges.

A second aim of this study was to determine whether strongly binding myosin crossbridges influence loaded shortening velocities and relative power output of cardiac myocytes. We investigated the hypothesis that strongly binding myosin crossbridges, as opposed to [Ca²⁺] per se, play the dominant role in determining power output in skinned cardiac myocytes. This idea was tested by characterizing power-load curves in single skinned cardiac myocyte preparations activated at fixed [Ca²⁺] before and after addition of NEM-S1. Finally, we investigated whether fixed force levels induced by Ca²⁺ alone or Ca²⁺ plus NEM-S1 produced differences in loading shortening speeds and power output in skinned cardiac myocyte preparations. These experiments addressed the possibility that strongly binding crossbridges influence loaded shortening velocities independent of force levels.

	Materials and Methods

Top Abstract Introduction Materials and Methods Results Discussion References

 
Cardiac Myocyte Preparation
Cardiac myocyte preparations were obtained by mechanical disruption of hearts from Sprague-Dawley rats as described previously.¹ The experimental apparatus for physiological measurements on myocyte preparations was similar to one previously described in detail.^{1 16}

Compositions of relaxing and activating solutions were as follows (in mmol/L): EGTA 7, free Mg²⁺ 1, imidazole 20, MgATP 4, creatine phosphate 14.5 (pH 7.0), various Ca²⁺ concentrations between 10^–9 (relaxing solution) and 10^–4.5 mol/L (maximal Ca²⁺-activating solution), and sufficient KCl to adjust ionic strength to 180 mmol/L.¹

NEM-S1
Myosin subfragment 1 (S1) was purified from rabbit fast-twitch skeletal muscle, modified with N-ethylmaleimide (NEM), and prepared as previously described.^{14 15}

Force-Velocity and Power-Load Measurements
All mechanical measurements were made at 12°C. For measurements of loaded shortening, a servosystem incorporating integrative feedback was used to control the load on the myocyte preparations as previously described.^{1 3}

To test the effects of NEM-S1 on loaded shortening, the myocyte was first maximally Ca²⁺ activated in pCa 4.5 solution to obtain an initial maximal force (ie, P_4.5), which was followed by a series of isotonic contractions (15). The myocyte was then transferred to a Ca²⁺-containing solution that yielded 40% P_4.5 (pCa solutions ranged between pCa 5.6 and 5.7), and a second series of isotonic contractions was performed. The myocyte was then bathed for 20 minutes in pCa 9.0 solution containing 6 µmol/L NEM-S1; ie, NEM-S1 was only added to pCa 9.0 solution. After NEM-S1 incubation, the myocyte was again transferred to an activating solution that yielded 40% P_4.5, and another series of isotonic contractions was performed. The pCa solutions yielding 40% P_4.5 after NEM-S1 treatment varied between preparations, ranging from pCa 9.0 to 5.7. A final series of isotonic contractions was obtained during Ca²⁺ activation in the identical pCa solution that yielded 40% P_4.5 before NEM-S1. Last, the myocyte preparation was transferred to a solution of pCa 4.5, and a final maximum force (P_4.5) was obtained.

Data Analysis
Myocyte length traces during isotonic contractions were fit with a single decaying exponential equation of the following form:

where L is cell length at time t, A and C are constants with dimensions of length, and k is the apparent rate constant of shortening (k_shortening). Velocity of shortening at any given time t was determined as the slope of a tangent to the fitted curve at that time point. For this study, velocities of shortening were calculated at t=0 ms.

Hyperbolic force-velocity curves were fit to force-velocity data using the Hill equation, and power-load curves were obtained by multiplying force x velocity as previously described.¹

One-way repeated-measures ANOVA was performed to determine whether there were significant effects of NEM-S1 on k_shortening or peak relative power output. Student-Newman-Keuls tests were used as post hoc tests to assess differences among means. For comparisons of k_shortening, values were used if myocytes shortened 5% to 15% of their original length. P<0.05 was chosen as indicating significance. All values are expressed as mean±SD.

	Results

Top Abstract Introduction Materials and Methods Results Discussion References

 
Myocyte Characteristics
Myocyte (n=7) dimensions and force characteristics were as follows: length, 149±52 µm; width, 22±3 µm; and maximum isometric force (P_4.5), 12±6 µN. The resting sarcomere length (SL) of these preparations was set to yield passive forces just slightly above 0 (SL2.25 µm). During maximal Ca²⁺ activation (ie, at pCa 4.5), mean SL shortened to 2.20 µm during fixed-end, isometric contractions, which is indicative of low end compliance of the myocytes at the points of attachment.

Effects of NEM-S1 on Loaded Shortening in Cardiac Myocytes
Loaded shortening varies as a function of activator [Ca²⁺] in skinned cardiac myocyte preparations, in that lower [Ca²⁺] yields slower shortening velocities and greater curvature in length traces during lightly loaded contractions^{1 2} (Figure 1). To investigate possible mechanisms by which Ca²⁺ regulates loaded shortening, isotonic contractions were recorded during constant [Ca²⁺] activations both before and after the addition of 6 µmol/L NEM-S1. This concentration of NEM-S1 was chosen on the basis of previous experiments, which showed that 6 µmol/L NEM-S1 does not alter peak Ca²⁺-activated force but does increase force at submaximal [Ca²⁺].^{14 15} At a given [Ca²⁺], NEM-S1 potentiated force and altered the time course of loaded shortening by reducing the curvature of length traces (Figure 2). To quantify curvature of length traces, apparent rate constants of shortening (ie, k_shortening) were calculated from exponentials fit to length traces before and after addition of NEM-S1. k_shortening values during submaximal activations decreased from 4.4±2.9 in controls (n=57) to 2.1±1.0 (n=62) in the presence of NEM-S1 (see inset, Figure 2). These results indicate that strongly bound crossbridges have a marked effect on the time course of loaded shortening even at the same [Ca²⁺], perhaps by maintaining thin filaments in the open or activated state.

View larger version (24K): [in this window] [in a new window]   Figure 1. Figure 1. Loaded shortening of myocytes at 2 Ca2+ concentrations. During maximal Ca2+ activation (pCa 4.5), the myocyte shortened at a nearly constant velocity during a 250-ms load clamp at 0.37 Po. When [Ca2+] was reduced to pCa 5.6 to yield 40% P4.5, the length trace was considerably more curvilinear during shortening at a nearly identical load of 0.33 Po.

View larger version (21K): [in this window] [in a new window]   Figure 2. Figure 2. Effect of NEM-S1 on loaded shortening in a submaximally activated myocyte. NEM-S1 increased steady force and reduced length trace curvature. During the load clamp, myocyte force was held at 0.33 and 0.30 Po in solutions of pCa 5.6 and of pCa 5.6 plus NEM-S1, respectively. In this example, the curvature of the fitted length trace (kshortening) fell from 6.3 to 2.0 when NEM-S1 was added. Inset, kshortening values (mean±SD) for 7 myocyte preparations during control (n=57) and NEM-S1 (n=62) activations in the same pCa solutions. *Significantly different compared with control.

The effects of NEM-S1 on curvature at constant [Ca²⁺] may result simply from NEM-S1–induced increases in force and presumably greater thin filament activation levels. To examine this idea and further investigate regulation of loaded shortening, isotonic contractions were next assessed during constant force levels before and after addition of NEM-S1. To obtain the same force (40% of P_4.5) in the presence and absence of NEM-S1, [Ca²⁺] had to be reduced during activations with NEM-S1. Specifically, Ca²⁺ concentrations in activating solutions were adjusted between 5.7 and as low as pCa 9.0 to obtain the same force as before NEM-S1 incubation. Even when force levels were equalized, NEM-S1 significantly reduced the curvature of length traces in submaximally activated myocytes (Figure 3). k_shortening values were 4.4±2.9 (n=62) in the absence of NEM-S1 compared with 2.4±1.2 (n=65) with NEM-S1 (Figure 3). Interestingly, the curvature of length traces obtained in the presence of NEM-S1 was similar to the curvature during isotonic contractions when the same myocytes were maximally Ca²⁺ activated (ie, pCa 4.5); that is, NEM-S1 eliminated the activation dependence of curvature in shortening records. During maximal Ca²⁺ activations (ie, pCa 4.5 solution), k_shortening was 1.8±1.1 (n=54). Given that thin filament activation levels are thought to be related to the amount of Ca²⁺ bound to troponin C and the number of strongly bound crossbridges to actin,⁴ high [Ca²⁺] would be predicted to maintain high levels of thin filament activation even during loaded shortening. NEM-S1 is also thought to maintain thin filaments at high activation levels during mechanical perturbations,¹⁴ although only a fraction of the length of each thin filament would presumably remain activated (ie, in the open state) in the presence of NEM-S1 at low [Ca²⁺], given that submaximal isometric forces are produced. The finding that curvature of shortening is similar during maximal Ca²⁺ activations and during submaximal activations in the presence of NEM-S1 is consistent with the idea that inactivation of the thin filament due to shortening-induced reductions in crossbridge number contributes to curvilinear shortening observed during submaximal isotonic contractions.

View larger version (21K): [in this window] [in a new window]   Figure 3. Figure 3. Effect of NEM-S1 on time course of loaded shortening when isometric force was kept constant by varying [Ca2+]. NEM-S1 reduced the curvature of the length trace even when the myocyte was activated with Ca2+ to the same force level. Here a loaded shortening trace is shown during a force clamp (0.33 Po) when the myocyte was activated in pCa 5.6 solution. A second length trace (load clamp=0.30 Po) from the same preparation was obtained after exposure to NEM-S1 and activation at pCa 6.1, which yielded the same force as pCa 5.6 without prior exposure to NEM-S1. Even at the same isometric force, NEM-S1 reduced the curvature of the length trace (control kshortening,=6.3; NEM-S1 kshortening=1.5). Inset, kshortening values (mean±SD) for 7 myocyte preparations during control (n=57) and NEM-S1 (n=65) activations to the same force levels. *Significantly different compared with control.

Effects of NEM-S1 on Force-Velocity and Power-Load Curves
A second aim of this study was to assess the role of [Ca²⁺] per se versus strongly binding crossbridges in determining loaded shortening velocities (at the onset of shortening) and relative power output (ie, power output normalized for isometric force) in skinned cardiac myocyte preparations. At a given [Ca²⁺], increasing the number of strongly binding crossbridges with NEM-S1 elevated isometric force, sped loaded shortening velocities, and thus increased relative power output at nearly all loads (Figure 4). Peak relative power output increased from 0.074±0.016 (n=7) before NEM-S1 to 0.107±0.027 (n=7) in the presence of NEM-S1 during activation in solutions of identical [Ca²⁺]. Force-velocity and relative power-load curves were also shifted upward by NEM-S1 even when [Ca²⁺] was adjusted to yield steady forces equal to values before NEM-S1 (Figure 5). Peak relative power output was 0.100±0.025 with NEM-S1 (n=7) compared with 0.074±0.016 (n=7) before NEM-S1. In fact, NEM-S1 eliminated all Ca²⁺ dependence of relative power output; ie, peak relative power output was the same during maximal Ca²⁺ activations (0.099±0.026) as during submaximal Ca²⁺ activations with NEM-S1 (inset Figure 5).

View larger version (28K): [in this window] [in a new window]   Figure 4. Figure 4. Cumulative force-velocity and power-load curves from myocyte preparations activated in the same pCa solution (either pCa 5.7 or pCa 5.6) before and after addition of NEM-S1. NEM-S1 significantly increased shortening velocity and power output at each submaximal load when myocytes were activated in the same pCa solution.

View larger version (35K): [in this window] [in a new window]   Figure 5. Figure 5. Cumulative force-velocity and power-load curves from myocytes that generated the same force before and after exposure to NEM-S1. NEM-S1 increased shortening velocity and power output at submaximal loads even when myocytes were Ca2+ activated to the same force. In the presence of NEM-S1, Ca2+-activating solutions were adjusted between pCa 9.0 and 5.7 to yield the same force (40% P4.5) as before treatment with NEM-S1. Inset, Peak relative power output (mean±SD) for myocyte preparations Ca2+ activated with and without NEM-S1. *Significantly different compared with 0.4 P4.5.

	Discussion

Top Abstract Introduction Materials and Methods Results Discussion References

 
This study examined potential mechanisms by which Ca²⁺ regulates loaded shortening velocity and relative power output in cardiac myocytes. To do this, we used a strongly bound crossbridge derivative, NEM-S1, which is thought to maintain thin filaments in the open state, allowing endogenous crossbridges to cycle through power-generating transitions. We found that both loaded shortening velocities and relative power output varied predominantly by addition of strongly binding crossbridges, as opposed to [Ca²⁺] per se. At a fixed [Ca²⁺], NEM-S1 increased force levels and markedly reduced the curvature of loaded shortening traces, sped loaded shortening, and increased relative power output at nearly all loads in skinned cardiac myocytes. Even when steady isometric force was kept constant by adjusting [Ca²⁺], NEM-S1 still reduced the curvature of length traces, sped loaded shortening, and increased relative power output. These findings indicate that strongly binding crossbridges modulate loaded shortening and power output and imply that reduced levels of thin filament activation due to decreased numbers of strongly binding crossbridges could mediate the slower shortening velocities reported previously in myocardial preparations activated at low [Ca²⁺].^{17 18 19 20} Because we were not able to directly measure thin filament activation levels during these experiments, our conclusion assumes that NEM-S1 maintains the thin filament in an open or activated state, which is based on previous reports that NEM-S1 activates acto-S1 ATPase activity in solution¹³ and force and kinetics of force development in skinned fibers.^{14 15}

The regulation of muscle contraction is thought to involve complex molecular processes involving both thin and thick filament proteins.²¹ Current models propose that thin filaments occupy 3 distinct states (ie, blocked, closed, and open) depending on the relative position of tropomyosin.^{5 6 7} In the Ca²⁺-bound closed state, there is increased probability that weakly bound crossbridges will make the transition to strongly bound, non–force-generating states. Such strongly bound, non–force-generating crossbridges are thought to move tropomyosin farther into the groove between actin monomers yielding the transition to the open state in which strongly bound, force-generating crossbridges accumulate. In the present study, we used NEM-S1 to potentially increase the probability of thin filaments occupying the open state even at low levels of myoplasmic Ca²⁺, to study the relative contributions of Ca²⁺ per se and strongly binding crossbridges in determining loaded shortening velocity and power output in cardiac myocytes. Previous work has shown that loaded shortening and power output increase with the levels of Ca²⁺ in skinned cardiac myocytes.² Ca²⁺ regulation could arise from direct effects of Ca²⁺ binding to alter crossbridge cycling kinetics and loaded shortening velocity, or Ca²⁺ might regulate velocity indirectly by affecting the level of thin filament activation by strongly binding crossbridges. Using NEM-S1 to alter the number of strongly bound crossbridges, our results show that Ca²⁺ likely regulates power output primarily by the second mechanism.

Several earlier studies found that both absolute and relative power output increased when the level of activation (as determined by force) was increased in intact cardiac muscle preparations.^{18 22 23} In those experiments, activation was varied in one of several ways, including inotropic agents such as ß-agonists, increased [Ca²⁺]_o, or measuring loaded shortening at various times during twitch contractions. Despite the consistent finding of Ca²⁺-induced alterations in power output, the subcellular factors that limit loaded shortening and relative power output in cardiac muscle remained uncertain. One possibility is that crossbridges must work against internal loads during shortening of cardiac muscle. The source of internal load is uncertain but may be the cytoskeleton,¹² titin/connectin,^{19 24} or crossbridges themselves.²⁵ A consequence of an internal load is that the proportion of cycling crossbridges required to overcome these loads is greater during submaximal than maximal Ca²⁺ activations. Consequently, proportionately fewer crossbridges would be available to work against an external load, so that the rate of crossbridge and muscle shortening will be slowed. A second factor that may also reduce the number of cycling crossbridges is cooperative inactivation of the thin filament as muscle shortens, as previously suggested.^{2 11} Because muscle shortening reduces the number of crossbridges in strongly bound states, there will be greater likelihood of thin filament transition from open to closed states based on the previously mentioned model of regulation. Cooperative inactivation of the thin filament will further reduce the proportion of cycling crossbridges bearing a relative external load, yielding even slower loaded shortening and decreased power output. Consistent with this idea, loaded shortening in the present study was faster and relative power output greater in the presence of strongly bound crossbridges (ie, NEM-S1) both when [Ca²⁺] was kept constant and when force-generating capability was kept constant by varying [Ca²⁺].

Our results also address the mechanisms by which the velocity of loaded shortening progressively slows during submaximal Ca²⁺ activations. We found that such curvilinear shortening was markedly reduced in the presence of NEM-S1 either at constant [Ca²⁺] or constant force-generating capabilities, a finding that is consistent with the idea that cooperative inactivation of the thin filament contributes to curvilinear shortening. However, other mechanisms might also contribute to curvilinear shortening. For example, a subpopulation of slowly detaching crossbridges has been proposed to arise in zones of the thin filaments between regions that are in closed and open states.²⁵ Experimental evidence for this idea came from the finding that unloaded shortening was biphasic during submaximal Ca²⁺ activation of skinned fast-twitch fibers; ie, shortening up to 40 to 80 nm per half-sarcomere was markedly faster than shortening beyond 80 nm per half-sarcomere in fast-twitch skeletal fibers. The idea here is that after shortening of 80 nm per half-sarcomere, the slowly detaching crossbridges in these transition zones become compressed and produce a load opposing shortening. The activation effects of NEM-S1 would be predicted to reduce the number of transition zones and thus the number of slowly detaching crossbridges. However, such a crossbridge-dependent internal load is unlikely to be the sole or even the main contributor to curvilinear shortening in cardiac muscle, because considerable curvature was observed in our length traces before sarcomeres had shortened 40 to 80 nm per half-sarcomere.

Physiological Significance
The ability of the heart to pump blood through the circulation depends on a number of factors, including preload, afterload, and inotropic state. The inotropic state of the heart, and thus its pumping capacity, is determined in large part by variations in myoplasmic [Ca²⁺]. Changes in [Ca²⁺] have marked effects on the rate of work produced by individual myocytes, which ultimately determines stroke volume. Results here suggest that [Ca²⁺]-induced changes in loaded shortening and power output are largely mediated by crossbridge number–induced changes in the level of thin filament activation. Thus, cooperative activation of the thin filament triggered by Ca²⁺ during isovolumetric contraction seems to be a major contributor to power produced by the myocardium during ejection and to stroke volume.

Another important functional aspect of the cardiac cycle is that peak ventricular and aortic pressures are reached before the end of ejection.²⁶ This arises because the rate that blood exits the aorta exceeds the rate of ventricular ejection, which is determined by myocardial power output. Therefore, the fall in ventricular pressure during the latter part of systole may be due, at least in part, to a slowing of loaded shortening velocity of cardiac myocytes, which is consistent with the progressive slowing of shortening in skinned cardiac myocyte preparations observed during submaximal Ca²⁺ activations. Evidence from the current study suggests that such slowing of shortening arises at least in part from progressive cooperative inactivation of the thin filaments throughout shortening.

	Acknowledgments

 

This study was supported by NIH Grant HL-57852 (to K.S.M.) and HL-54581 (to R.L.M). We thank Dr Daniel P. Fitzsimons and Chad Warren for assistance in preparation of NEM-S1.

Received May 16, 2000; revision received August 17, 2000; accepted August 28, 2000.

	References

Top Abstract Introduction Materials and Methods Results Discussion References

 

1. McDonald KS, Wolff MR, Moss RL. Force-velocity and power-load curves in rat skinned cardiac myocytes. J Physiol. 1998;519–531.
2. McDonald KS. Ca²⁺ dependence of loaded shortening in rat skinned cardiac myocytes and skeletal muscle fibers. J Physiol. 2000;525:169–181.[Abstract/Free Full Text]
3. Sweitzer NK, Moss RL. Determinants of loaded shortening velocity in single cardiac myocytes permeabilized with -hemolysin. Circ Res. 1993;73:1150–1162.[Abstract/Free Full Text]
4. Gordon AM, Homsher E, Regnier M. Regulation of contraction in striated muscle. Physiol Rev. 2000;80:853–924.[Abstract/Free Full Text]
5. McKillop DF, Geeves MA. Regulation of the interaction between actin and myosin subfragment 1: evidence for three states of the thin filament. Biophys J. 1993;65:693–701.[Abstract/Free Full Text]
6. Holmes KC. The actomyosin interaction and its control by tropomyosin. Biophys J. 1995;68:2s–7s.
7. Wahr PA, Metzger JM. Role of Ca²⁺ and cross-bridges in skeletal muscle thin filament activation probed by Ca²⁺ sensitizers. Biophys J. 1999;76:2166–2176.[Abstract/Free Full Text]
8. Julian FJ, Sollins MR. Variation of muscle stiffness with force at increasing speeds of shortening. J Gen Physiol. 1975;66:287–302.[Abstract/Free Full Text]
9. Ford LE, Huxley AF, Simmons RM. Tension transients during steady shortening of frog muscle fibres. J Physiol. 1985;361:131–150.[Abstract/Free Full Text]
10. Solaro RJ, Van Eyk J. Altered interactions among thin filament proteins modulate cardiac function. J Mol Cell Cardiol. 1996;28:217–230.[Medline] [Order article via Infotrieve]
11. Iwamoto I. Thin filament cooperativity as a major determinant of shortening velocity in skeletal muscle fibers. Biophys J. 1998;74:1452–1464.[Abstract/Free Full Text]
12. Brenner B. The necessity of using two parameters to describe isotonic shortening velocity of muscle tissues: the effect of various interventions upon initial shortening velocity (v_i) and curvature (b). Basic Res Cardiol. 1986;81:54–69.[Medline] [Order article via Infotrieve]
13. Williams DL, Greene LE, Eisenberg E. Cooperative turning on of myosin subfragment 1 ATPase by the Tn-Tm-Actin complex. Biochemistry. 1988;27:6987–6993.[Medline] [Order article via Infotrieve]
14. Swartz DR, Moss RL. Influence of a strong-binding myosin analog on calcium-sensitive mechanical properties of skinned skeletal muscle fibers. J Biol Chem. 1992;267:20497–20506.[Abstract/Free Full Text]
15. Fitzsimons DP, Moss RL. Strong binding of myosin modulates length-dependent Ca²⁺ activation of rat ventricular myocytes. Circ Res. 1998;83:602–607.[Abstract/Free Full Text]
16. Moss RL. Sarcomere length-tension relations of frog skinned muscle fibres during calcium activation at short lengths. J Physiol. 1979;292:177–202.[Abstract/Free Full Text]
17. Maughan DW, Low ES, Alpert NR. Isometric force development, isotonic shortening, and elasticity measurements from Ca²⁺-activated ventricular muscle of the guinea pig. J Gen Physiol. 1978;71:431–451.[Abstract/Free Full Text]
18. Chiu YC, Walley KR, Ford LE. Comparison of the effects of different inotropic interventions of force, velocity, and power in rabbit myocardium. Circ Res. 1989;65:1161–1171.[Abstract/Free Full Text]
19. De Tombe PP, ter Keurs EDJ. An internal viscous element limits unloaded velocity of sarcomere shortening in rat cardiac trabeculae. J Physiol. 1992;454:619–642.[Abstract/Free Full Text]
20. Hofmann PA, Moss RL. Effects of calcium on shortening velocity in frog chemically skinned atrial myocytes and in mechanically disrupted ventricular myocardium from rat. Circ Res. 1992;70:885–892.[Abstract/Free Full Text]
21. Solaro RJ, Rarick HM. Troponin and tropomyosin: proteins that switch on the tune in the activity of cardiac myofilaments. Circ Res. 1998;83:471–480.[Abstract/Free Full Text]
22. Sonnenblick EH. Force-velocity relations in mammalian heart muscle. Am J Physiol. 1962;202:931–939.
23. Brutsaert DL, Claes VA, Goethals MA. Effect of calcium on force-velocity-length relations of heart muscle of the cat. Circ Res. 1973;32:385–392.[Abstract/Free Full Text]
24. Granzier HL, Irving TC. Passive tension in cardiac muscle: contribution of collagen, titin, microtubules, and intermediate filaments. Biophys J. 1995;68:1027–1044.[Abstract/Free Full Text]
25. Moss RL. Ca²⁺ regulation of mechanical properties of striated muscle: mechanistic studies using extraction and replacement of regulatory proteins. Circ Res. 1992;70:865–884.[Abstract/Free Full Text]
26. Katz AM. Physiology of the Heart. New York, NY: Raven Press; 1992:361–364.

This article has been cited by other articles:

				L. M. Hanft, F. S. Korte, and K. S. McDonald Cardiac function and modulation of sarcomeric function by length Cardiovasc Res, March 1, 2008; 77(4): 627 - 636. [Abstract] [Full Text] [PDF]

				J. T. Pearson, M. Shirai, H. Tsuchimochi, D. O. Schwenke, T. Ishida, K. Kangawa, H. Suga, and N. Yagi Effects of Sustained Length-Dependent Activation on In Situ Cross-Bridge Dynamics in Rat Hearts Biophys. J., December 15, 2007; 93(12): 4319 - 4329. [Abstract] [Full Text] [PDF]

				F. S. Korte and K. S. McDonald Sarcomere length dependence of rat skinned cardiac myocyte mechanical properties: dependence on myosin heavy chain J. Physiol., June 1, 2007; 581(2): 725 - 739. [Abstract] [Full Text] [PDF]

				A. C. Hinken and R. J. Solaro A Dominant Role of Cardiac Molecular Motors in the Intrinsic Regulation of Ventricular Ejection and Relaxation Physiology, April 1, 2007; 22(2): 73 - 80. [Abstract] [Full Text] [PDF]

				A. C. Hinken and K. S. McDonald Inorganic phosphate speeds loaded shortening in rat skinned cardiac myocytes Am J Physiol Cell Physiol, August 1, 2004; 287(2): C500 - C507. [Abstract] [Full Text] [PDF]

				K. S. McDonald and T. J. Herron It Takes "Heart" to Win: What Makes the Heart Powerful? Physiology, October 1, 2002; 17(5): 185 - 190. [Abstract] [Full Text] [PDF]

				D. Fatkin and R. M. Graham Molecular Mechanisms of Inherited Cardiomyopathies Physiol Rev, October 1, 2002; 82(4): 945 - 980. [Abstract] [Full Text] [PDF]

				W. G. Pyle, M. P. Sumandea, R. J. Solaro, and P. P. De Tombe Troponin I serines 43/45 and regulation of cardiac myofilament function Am J Physiol Heart Circ Physiol, September 1, 2002; 283(3): H1215 - H1224. [Abstract] [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 McDonald, K. S. Articles by Moss, R. L. Search for Related Content PubMed PubMed Citation Articles by McDonald, K. S. Articles by Moss, R. L. Related Collections Structure Contractile function Calcium cycling/excitation-contraction coupling