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

2006; Lippincott Williams & Wilkins; Volume: 98; Issue: 4 Linguagem: Inglês

10.1161/01.res.0000214324.34563.31

1524-4571

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

Neuroscience and Neuropharmacology Research

HomeCirculation ResearchVol. 98, No. 4A Novel Mechanism of Pacemaker Control That Depends on High Levels of cAMP and PKA-Dependent Phosphorylation Free AccessEditorialPDF/EPUBAboutView PDFView EPUBSections ToolsAdd to favoritesDownload citationsTrack citationsPermissions ShareShare onFacebookTwitterLinked InMendeleyReddit Jump toFree AccessEditorialPDF/EPUBA Novel Mechanism of Pacemaker Control That Depends on High Levels of cAMP and PKA-Dependent PhosphorylationA Precisely Controlled Biological Clock John H.B. Bridge, Christopher J. Davidson and Eleonora Savio-Galimberti John H.B. BridgeJohn H.B. Bridge From The Nora Eccles Harrison Cardiovascular Research and Training Institute (CVRTI), University of Utah, Salt Lake City. , Christopher J. DavidsonChristopher J. Davidson From The Nora Eccles Harrison Cardiovascular Research and Training Institute (CVRTI), University of Utah, Salt Lake City. and Eleonora Savio-GalimbertiEleonora Savio-Galimberti From The Nora Eccles Harrison Cardiovascular Research and Training Institute (CVRTI), University of Utah, Salt Lake City. Originally published3 Mar 2006https://doi.org/10.1161/01.RES.0000214324.34563.31Circulation Research. 2006;98:437–439The 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: Ikr, If, Ist, ICa (with two components: ICa-T and ICa-L), and INCX.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.5Dr 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 HypothesisDr 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 (INCX) which contributes to DD. INCX, 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 ClockVinogradova 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). Download figureDownload PowerPointKey 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+ contentThe 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 ICa. 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 (Po) of the RyR by an allosteric mechanism involving a complex of SR proteins. This increase in Po 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 Po 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.The opinions expressed in this editorial are not necessarily those of the editors or of the American Heart Association.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.FootnotesCorrespondence 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 [email protected] References 1 Irisawa H, Brown HF, Giles W. Cardiac Pacemaking in the Sinoatrial Node. Physiol Rev. 1993; 73: 197–227.CrossrefMedlineGoogle Scholar2 Vinogradova TM, Lyashkov AE, Zhu W, Ruknudin AM, Sirenko S, Yang D, Deo Sh, Barlow M, Johnson Sh, Caffrey JL, Zhou YY, Xiao RP, Cheng H, Stern MD, Maltsev VA, Lakatta EG. High basal protein kinase a dependent phosphorylation drives rhythmic internal Ca2+ store oscillations and spontaneous beating of cardiac pacemaker cells. Circ Res. 2006; 98: 505–514.LinkGoogle Scholar3 Guo J, Ono K, Noma A. A sustained inward current activated at the diastolic potential range in rabbit sino-atrial node cells. J Physiol. 1995; 483: 1–13.MedlineGoogle Scholar4 Bogdanov KY, Vinogradova TM, Lakatta EG. Sinoatrial nodal cell ryanodine receptor and Na(+)-Ca(2+) exchanger: molecular partners in pacemaker regulation. Circ Res. 2001; 88: 1254–1258.CrossrefMedlineGoogle Scholar5 Vinogradova TM, Maltsev VA, Bogdanov KY, Lyashkov AE, Lakatta EG. Rhythmic Ca2+ oscillations drive sinoatrial nodal cell pacemaker function to make the heart tick. Ann N Y Acad Sci. 2005; 1047: 138–156.CrossrefMedlineGoogle Scholar6 Inoue M, Bridge JH. Ca2+ sparks in rabbit ventricular myocytes evoked by action potentials: involvement of clusters of L-type Ca2+ channels. Circ Res. 2003; 92: 532–538.LinkGoogle Scholar7 Vinogradova TM, Zhou YY, Maltsev V, Lyashkov A, Stern M, Lakatta EG. Rhythmic ryanodine receptor Ca2+ releases during diastolic depolarization of sinoatrial pacemaker cells do not require membrane depolarization. Circ Res. 2004; 94: 802–809.LinkGoogle Scholar8 Maltsev VA, Vinogradova TM, Bogdanov KY, Lakatta EG, Stern MD. Diastolic calcium release controls the beating rate of rabbit sinoatrial node cells: numerical modeling of the coupling process. Biophys J. 2004; 86: 2596–2605.CrossrefMedlineGoogle Scholar9 Stuyvers BD, Dun W, Matkovich S, Sorrentino V, Boyden PA, Ter Keurs. Ca2+ Sparks and waves in canine purkinje cells: a triple layered system of Ca2+ activation. Circ Res. 2005; 97: 35–43.LinkGoogle Scholar10 Gyorke I, Hester N, Jones LR, Gyorke S. The role of calsequestrin, triadin, and junctin in conferring cardiac ryanodine receptor responsiveness to luminal calcium. Biophys J. 2004; 86: 2121–2128.CrossrefMedlineGoogle Scholar11 Trafford AW, Diaz ME, Sibbring GC, Eisner DA. Modulation of CICR has no maintained effect on systolic Ca2+: simultaneous measurements of sarcoplasmic reticulum and sarcolemmal Ca2+ fluxes in rat ventricular myocytes. J Physiol. 2000; 522: 259–270.CrossrefMedlineGoogle Scholar12 Brown HF, Kimura J, Noble D, Noble SJ, Taupignon A. The ionic currents underlying pacemaker activity in rabbit sino-atrial node: experimental results and computer simulations. Proc R Soc Lond B Biol Sci. 1984; 222: 329–347.CrossrefMedlineGoogle Scholar13 Lancaster MK, Jones SA, Harrison SM, Boyett MR. Intracellular Ca2+ and pacemaking within the rabbit sinoatrial node: heterogeneity of role and control. J Physiol. 2004; 556: 481–494.CrossrefMedlineGoogle Scholar eLetters(0)eLetters should relate to an article recently published in the journal and are not a forum for providing unpublished data. Comments are reviewed for appropriate use of tone and language. Comments are not peer-reviewed. Acceptable comments are posted to the journal website only. Comments are not published in an issue and are not indexed in PubMed. Comments should be no longer than 500 words and will only be posted online. References are limited to 10. Authors of the article cited in the comment will be invited to reply, as appropriate.Comments and feedback on AHA/ASA Scientific Statements and Guidelines should be directed to the AHA/ASA Manuscript Oversight Committee via its Correspondence page.Sign In to Submit a Response to This Article Previous Back to top Next FiguresReferencesRelatedDetailsCited BySicouri S, Blazek J, Belardinelli L and Antzelevitch C (2012) Electrophysiological Characteristics of Canine Superior Vena Cava Sleeve Preparations, Circulation: Arrhythmia and Electrophysiology, 5:2, (371-379), Online publication date: 1-Apr-2012. Maltsev V and Lakatta E (2009) Synergism of coupled subsarcolemmal Ca 2+ clocks and sarcolemmal voltage clocks confers robust and flexible pacemaker function in a novel pacemaker cell model , American Journal of Physiology-Heart and Circulatory Physiology, 10.1152/ajpheart.01118.2008, 296:3, (H594-H615), Online publication date: 1-Mar-2009. Chorvat D and Chorvatova A (2008) Cardiac cell: a biological laser?, Biosystems, 10.1016/j.biosystems.2007.11.003, 92:1, (49-60), Online publication date: 1-Apr-2008. Kizana E, Ginn S, Smyth C, Boyd A, Thomas S, Allen D, Ross D and Alexander I (2006) Fibroblasts modulate cardiomyocyte excitability: implications for cardiac gene therapy, Gene Therapy, 10.1038/sj.gt.3302813, 13:22, (1611-1615), Online publication date: 1-Nov-2006. March 3, 2006Vol 98, Issue 4 Advertisement Article InformationMetrics https://doi.org/10.1161/01.RES.0000214324.34563.31PMID: 16514072 Originally publishedMarch 3, 2006 Keywordslocal calcium releaseryanodine receptorNa–Ca exchangerdiastolic depolarizationsarcoplasmic reticulumPDF download Advertisement