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(Circulation Research. 2006;98:437.)
© 2006 American Heart Association, Inc.

Editorials

A Novel Mechanism of Pacemaker Control That Depends on High Levels of cAMP and PKA-Dependent Phosphorylation

A Precisely Controlled Biological Clock

John H.B. Bridge, Christopher J. Davidson, Eleonora Savio-Galimberti

From The Nora Eccles Harrison Cardiovascular Research and Training Institute (CVRTI), University of Utah, Salt Lake City.

Correspondence to John H.B. Bridge, PhD, Research Professor of Internal Medicine, The Nora Eccles Harrison, Cardiovascular Research and Training Institute (CVRTI), University of Utah, 95 S 2000 E Back, Salt Lake City, UT. E-mail bridge{at}cvrti.utah.edu

See related article, pages 505–514

Key Words: local calcium release • sarcoplasmic reticulum • Na–Ca exchanger • ryanodine receptor • diastolic depolarization

	Introduction

Top Introduction The Hypothesis How Does NCX Produce... Relationship between the LCR... References

 
The mammalian heart has remarkable intrinsic rhythmic properties. It is widely agreed that spontaneous diastolic depolarizations (DDs) in sino-atrial node cells (SANCs) periodically initiate action potentials (AP), which set the rhythm of the heart.¹ Efforts to understand the origin of the pacemaker activity have a lengthy history and the subject has, for various technical reasons, proved somewhat intractable.

Any explanation of pacemaker activity must address three central issues. First, how do DDs arise? Second, what determines their periodicity? And third, how is the rate modulated? In this issue of Circulation Research, Vinogradova and her colleagues² offer some novel observations that go far toward explaining these issues. The article, which is the most recent of an exhaustive series of experiments from Dr Lakatta’s group, offers an explanation of the control of pacemaker activity based on both biophysical and biochemical observations, integrated with appropriate mathematical modeling (see supplement). This work depends on the central idea that pacemaking involves complex interactions within a multi-molecular complex that resides in both sarcolemmal and SR membranes. An attractive feature of this work is that it suggests a number of interesting structural and functional avenues of investigation that are amenable to contemporary biophysical methods, particularly confocal microscopy.

No single current by itself is responsible for DD. It is the sum of at least 6 ionic currents: I_kr, I_f, I_st, I_Ca (with two components: I_Ca-T and I_Ca-L), and I_NCX.^1,3 In a previous study, Bogdanov et al⁴ show that sodium–calcium exchanger (NCX) is of crucial importance to maintaining pacemaker activity. A more complete discussion of the temporal relationships between these various currents is reviewed elsewhere.⁵

Dr Lakatta’s group have emphasized the importance of the involvement of intracellular Ca²⁺ (particularly SR Ca²⁺) in the regulation of pacemaker activity. Although their work is extremely provocative, it is worth pointing out that this view of pacemaker activity is not unanimous. It is in principle possible to obtain pacemaker activity with only three time- and voltage-dependent currents.¹² The implication of this is that Ca²⁺ homeostasis need not be involved in pacemaker activity. Moreover, recently Lancaster et al¹³ have pointed out that smaller centrally located SANCs continue to pace in the presence of ryanodine. Clearly the involvement of Ca homeostasis in pacemaker activity is controversial.

	The Hypothesis

Top Introduction The Hypothesis How Does NCX Produce... Relationship between the LCR... References

 
Dr Lakatta’s group has developed the idea that in SANCs there are periodic spontaneous local Ca²⁺ release events (LCRs), through RyR₂ which cause a rise in Ca²⁺ in a domain that is closely opposed to NCX. These exchangers preferentially extrude Ca²⁺ which produces an inward current (I_NCX) which contributes to DD. I_NCX, although not the only cause of DDs, clearly augments the later part of DD. In an earlier publication Vinogradova et al⁷ showed that the Fast Fourier Transform of membrane current fluctuations displayed a similar periodicity to LCRs, with a dominant power at 2.9 Hz. The key finding of the current article is that this mechanism depends crucially on a high level of basal cAMP and its attendant PKA phosphorylation. These high levels of cAMP seem, at least within the heart, to be characteristic of SANCs but not of ventricular cells.

	How Does NCX Produce Periodic Activity: The LCR Clock

Top Introduction The Hypothesis How Does NCX Produce... Relationship between the LCR... References

 
Vinogradova et al propose in this study that the LCR period functions as a clock during spontaneous beating. The period of this clock is the time from the onset of triggered SR Ca²⁺ release during the prior AP to LCR onset during subsequent DDs.² We begin our consideration of the LCR clock with the LCR events depicted in the Figure. This release from the SR is imaged with line scans along the longitudinal axis and beneath the sarcolemma of the cell (see Figure 3c of reference ²).

View larger version (23K): [in this window] [in a new window]   Key events and associated proteins of the pacemaker clock in sinoatrial nodal cells (SANC). *Targets for phosphorylation by PKA.

These local Ca²⁺ release events appear to be larger than sparks in rabbit cardiac myocytes.⁶ They display a temporal separation from the putative global release event which appears to be evoked during the AP. The LCRs are abolished by ryanodine but remain under voltage-clamp and in skinned cells. Bogdanov et al⁴ showed that LCRs appeared as sub-microscopic wavelike patterns in SANCs. These LCRs may resemble the small Ca²⁺ wavelets that Stuyvers et al⁹ detected in canine Purkinje cells. The LCRs studied by Vinogradova et al² do not seem to be necessarily responsible for triggering the putative global release events for reasons that are not clear but may be related to geometric and structural constraints. LCR events clearly warrant further investigation.

In the cascade that comprises the clock the next event of importance is the activation of NCX. Maltsev et al⁸ have calculated that these LCRs activate sufficient DD to ensure that an AP is evoked. The rate of this depolarization will depend on the magnitude of LCRs and as such is dependent on SR Ca²⁺ content.

	Relationship between the LCR and SR Ca2+ content

Top Introduction The Hypothesis How Does NCX Produce... Relationship between the LCR... References

 
The current article provides evidence that phosphorylation of numerous control points modulates the clock periodicity. We will now consider the involvement of the SR as one of the control points. Ca²⁺ is principally extruded from the cell by NCX. SR Ca²⁺ content will then depend mainly on the balance between the extrusion of Ca²⁺ and Ca²⁺ influx through L-type Ca²⁺ channels during each duty cycle. In particular, the integrated flux through local L-type Ca²⁺ channels depends on the extent of activation and inactivation of I_Ca. If in successive beats net influx increases, SR content will also increase, with reasonably predictable effects on the clock. The studies by Vinogradova et al² seem to imply that SR content and hence the magnitude and properties of LCRs are controlled exquisitely. How can SR content control LCRs? It seems that the finding by these authors that high basal cAMP and PKA-dependent phosphorylation in SANCs may suggest that SR Ca²⁺ content is somewhat high in these cells during normal activity. Most recently Gyorke et al¹⁰ have suggested that the concentration of luminal calcium influences the open probability (P_o) of the RyR by an allosteric mechanism involving a complex of SR proteins. This increase in P_o will on average increase the chances that a RyR opens with a shorter latency. Changes in RyR latency provide another point where the timing of the clock can be modified. Moreover, with an increased P_o and possibly elevated luminal Ca²⁺, the increase in SR permeability and the increased driving force for SR Ca²⁺ release could enlarge LCRs.

The studies depicted in this article invite quantitative consideration of the relationship between SR Ca²⁺ content and the size of the Ca²⁺ transient, which presumably includes LCR events. Trafford et al¹¹ have suggested that the relationship between Ca²⁺ release and SR Ca²⁺ content is extremely steep such that the Ca²⁺ transient is proportional to the 6th power of SR Ca²⁺ content. This means that if, as a result of the extent of phosphorylation suggested by this study, the SR Ca²⁺ content is high in SANCs, small changes in SR Ca²⁺ content would have rather large effects on the magnitude of LCRs. For this reason, the modification of the clock periodicity will be dependent on SR Ca²⁺ content and the magnitude and timing of the LCRs (supplemental Figure, available online at http://circres.ahajournals.org). There are a number of implications to these assumptions. First, small changes in transmembrane fluxes can have significant effects on the clock without a high metabolic cost in Ca²⁺ pumping. Secondly, the dynamic range of the clock can, in principle, be significantly influenced by small changes in the extent of phosphorylation at the various control points of the clock. Third, because clock function may depend on SR Ca²⁺ content, dramatic reductions in SR Ca²⁺ content would seriously disrupt the function of the clock. These reductions are unlikely because they would require large imbalances in sarcolemmal Ca²⁺ fluxes.

Vinogradova et al² also indicate that both phospholamban phosphorylation at serine 16 and phosphorylation on the RyRs is increased. The former will increase SR Ca²⁺ uptake. The latter will increase the Ca²⁺ leak through RyRs. Thus it is possible that SR Ca²⁺ content does not increase. How will this modify LCR generation? If Ca²⁺ accumulates in an unstirred layer adjacent to the RyRs, it might reach a threshold value where regenerative responses of RyR aggregates could produce LCRs. However, this increased cycling of Ca²⁺ through the SR would be at an increased energetic cost to pacemaker activity. It is clear that the entire issue of SR content in SANCs may be of considerable functional significance. As such, it may become central to future studies.

	Acknowledgments

 
The authors are grateful for the support of the National Institutes of Health Research grants No. HL62690 and HL70828. We also appreciate the continuing support of the Nora Eccles Treadwell Foundation.

	Footnotes

 
The opinions expressed in this editorial are not necessarily those of the editors or of the American Heart Association.

	References

Top Introduction The Hypothesis How Does NCX Produce... Relationship between the LCR... References

 

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Related Article:

High Basal Protein Kinase A–Dependent Phosphorylation Drives Rhythmic Internal Ca²⁺ Store Oscillations and Spontaneous Beating of Cardiac Pacemaker Cells

Tatiana M. Vinogradova, Alexey E. Lyashkov, Weizhong Zhu, Abdul M. Ruknudin, Syevda Sirenko, Dongmei Yang, Shekhar Deo, Matthew Barlow, Shavsha Johnson, James L. Caffrey, Ying-Ying Zhou, Rui-Ping Xiao, Heping Cheng, Michael D. Stern, Victor A. Maltsev, and Edward G. Lakatta
Circ. Res. 2006 98: 505-514. [Abstract] [Full Text] [PDF]

This Article Extract 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 Google Scholar Google Scholar Articles by Bridge, J. H.B. Articles by Savio-Galimberti, E. Search for Related Content PubMed PubMed Citation Articles by Bridge, J. H.B. Articles by Savio-Galimberti, E. Related Collections Related Article