The membrane and Ca2+ clock mechanisms of sinoatrial node automaticity are likely interdependent, contributing distinctly to short-term and long-term regulation of pacemaker rate.
Editorial FocusThe long and short of calcium-dependent automaticity in the sinoatrial nodeRichard B. RobinsonRichard B. RobinsonDepartment of Pharmacology and Center for Molecular Therapeutics, Columbia University, New York, New YorkPublished Online:01 Jan 2011https://doi.org/10.1152/ajpheart.01083.2010This is the final version - click for previous versionMoreSectionsPDF (43 KB)Download PDF ToolsExport citationAdd to favoritesGet permissionsTrack citations debate on the ionic basis of automaticity in the sinoatrial node (SAN) has persisted since the onset of the field of cardiac cellular electrophysiology, with the favored theory replaced or refined periodically (see Refs. 1, 16, 17, 21, 24–26). For many years, the accepted concept was that of a decaying repolarizing K+ current (IK2) against the background of a constant inward current (22). In the 1980s this was disproven although the role of rapid delayed rectifier K+ current (IKr) deactivation has been explored (5, 14) and replaced by a dominant role for a time-dependent, hyperpolarization-activated inward current (If) (6, 7, 10). In subsequent decades, various other inward membrane currents have been put forth as contributors or dominant players in cardiac automaticity (11, 23–25).Most recently, a strong argument has been made for a critical if not exclusive role of the Na+/Ca2+ exchanger (2, 17). The idea of Ca2+ homeostasis and the Na+/Ca2+ exchanger being involved in SAN automaticity is not new (3, 13, 29). However, the most recent variation of this proposal, typically referred to as the "Ca2+ clock" mechanism, greatly minimizes the role of membrane channels in initiating SAN automaticity or modulating rate in response to adrenergic stimulation. Instead, spontaneous and cyclical local Ca2+ release from the sarcoplasmic reticulum (SR) is the primary mechanism driving both basal and adrenergically stimulated rate, with membrane channels having at best a minor modulatory role. In this model, the depletion of the Ca2+ source driving the Na+/Ca2+ exchanger would be expected to result in the immediate cessation of automaticity.This has led to a spirited debate in the literature between the "membrane" (most often If) and "Ca2+ clock" advocates (16), a debate that can create the impression that these are largely independent and mutually exclusive mechanisms. Proponents of a major role of If cite the fact that the hyperpolarization-activated cyclic nucleotide-gated (HCN) gene family (the molecular correlate of If) is highly expressed in the SAN and other automatic tissues, that human mutations in HCN4 are associated with sinus rhythm abnormalities, and that the selective If blocker ivabradine slows sinus rate in humans (see Refs. 1, 8, 9). Proponents of the Ca2+ clock mechanism cite the fact that If blockers only slow but do not stop automaticity, whereas ryanodine, which depletes SR Ca2+ stores, can result in a complete cessation of automaticity. Also cited are reports that the rate response to adrenergic agonists is markedly attenuated following ryanodine. Confounding this interpretation is the observation that different laboratories find a wide range of results with ryanodine in isolated SAN cells, varying from no effect to full cessation of automaticity. When a more modest effect is observed, Ca2+ clock proponents argue that the drug was not used at a sufficiently high concentration or for a sufficiently long time period (see Refs. 15, 17, 18).What these latter arguments do not always consider, beyond the potential nonspecific effects of high doses of ryanodine (19), is that 1) sustained Ca2+ depletion may result in secondary effects on cellular homeostasis beyond simply eliminating an immediate Ca2+ source to drive the Na+/Ca2+ exchanger and that 2) some of these secondary effects may in turn impact membrane channel function. In other words, the "membrane" and "Ca2+ clock" mechanisms may be interdependent, and some of the apparent Ca2+ dependence may be indirect. Recent observations (20, 28) that the SAN expresses a Ca2+-stimulated adenylyl cyclase isoform (AC1 or AC8), rather than the typical cardiac isoform (AC5 or AC6), provide support for such interdependence. In fact, when Ca2+ is depleted with ryanodine, not only is the isoproterenol effect on rate greatly reduced (supporting the Ca2+ clock mechanism), but the isoproterenol effect on If is also lost, but not that of membrane permeable cAMP (4), arguing for an effect of Ca2+ homeostasis on adrenergic signaling and subsequent If responsiveness and thus supporting the interdependence of the two mechanisms. Also relevant are reports indicating that the inhibition of CaMKII reduces L-type Ca2+ current and automaticity and that CaMKII activity is dependent on local Ca2+ release (27).The article by Himeno et al. (12) in this issue of the American Journal of Physiology-Heart and Circulatory Physiology reexamines the question of Ca2+ dependence of SAN automaticity using a fresh approach. Here the emphasis is on rapid (within a few seconds) depletion of intracellular Ca2+, thereby providing the possibility of separating direct and secondary effects of disrupted Ca2+ homeostasis. In addition, they compare their experimental observations with the predictions of two computer models, one reliant on membrane channels (M model) to drive automaticity and the other dependent on local Ca2+ release driving the Na+/Ca2+ exchanger (C model).As one might expect, the C model predicts that Ca2+ chelation results in a rapid suppression of automaticity, whereas the M model predicts no effect of chelation on pacemaking. They then used 10 mM BAPTA in the pipet (rather than the more common introduction of membrane permeable BAPTA-AM) to rapidly reduce cytosolic Ca2+, resulting in the cessation of contraction, but not spontaneous activity, within a few seconds. High doses of SR blockers also failed to acutely alter pacemaking despite rapidly eliminating contractions. Taken together, these results argue against the Ca2+ clock mechanism being the major driver of automaticity on a beat to beat basis. However, the authors also acknowledge that, when cytosolic Ca2+ is depleted for an extended period, pacemaking is affected; automaticity ceased after ∼5 min of BAPTA dialysis through the pipet. While this is not the focus of the study by Himeno et al. (12) and the observation is not pursued, this result raises the intriguing possibility that sustained cytosolic Ca2+ depletion leads to other effects that impact automaticity, possibly involving L-type Ca2+ current rundown induced secondarily to the inhibition of CaMKII and/or the effects on the Ca2+-stimulated AC.There are some caveats, however. A key question is whether the delivery of BAPTA through the pipet, even if it abolishes visible contractions, truly eliminates Ca2+ transients just under the membrane. The authors argue that it does both from a mathematical consideration of the distances involved and from the observations that L-type inactivation kinetics and action potential duration both are rapidly affected by BAPTA delivery, suggesting the submembrane Ca2+ concentration is altered. However, a direct measure of Ca2+ transients just under the membrane is lacking and needs to be conducted to fully validate the authors' interpretation. Another limitation is that all the experiments were carried out in guinea pig SAN cells, and one should not assume that a comparable result will be seen in all species; abundant data exist suggesting differences in the details of ionic current contributions to SAN automaticity across species, including rabbit, mouse and canine. Thus the approach of Himeno et al. (12), focusing on the acute response to rapid Ca2+ depletion, should be replicated in SAN cells from additional species, including human tissue or tissue from animals where the basal heart rate is similar to that of humans.In summary, many researchers would probably agree that both the membrane and Ca2+ clock hypotheses are oversimplifications and that SAN automaticity reflects a mixed contribution from both mechanisms, with the debate reducing to that of the relative contribution of each. However, what the present study by Himeno et al. (12) may do is help redefine that debate by demonstrating that there may be distinct pathways by which Ca2+ depletion acutely (i.e., directly) and chronically (i.e., indirectly) impacts automaticity. Thus, rather than arguing about which is the "dominant" contributor to automaticity, we may instead begin to address how each contributes to short-term versus long-term regulation of pacemaker rate and how, over time, changes to one pathway impinge on the function of the other pathway.DISCLOSURESNo conflicts of interest, financial or otherwise, are declared by the author(s).REFERENCES1. Baruscotti M, Barbuti A, Bucchi A. The cardiac pacemaker current. J Mol Cell Cardiol 48: 55–64, 2010.Crossref | PubMed | ISI | Google Scholar2. Bogdanov KY, Vinogradova TM, Lakatta EG. Sinoatrial nodal cell ryanodine receptor and Na+-Ca2+ exchanger: molecular partners in pacemaker regulation. Circ Res 88: 1254–1258, 2001.Crossref | PubMed | ISI | Google Scholar3. 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 222: 329–347, 1984.Crossref | PubMed | ISI | Google Scholar4. Bucchi A, Baruscotti M, Robinson RB, DiFrancesco D. If-dependent modulation of pacemaker rate mediated by cAMP in the presence of ryanodine in rabbit sino-atrial node cells. J Mol Cell Cardiol 35: 905–913, 2003.Crossref | PubMed | ISI | Google Scholar5. Clark RB, Mangoni ME, Lueger A, Coutte B, Nargeot J, Giles WR. A rapidly-activating delayed rectifier K+ current, IKr, regulates pacemaker activity in adult mouse sinoatrial node cells. Am J Physiol Heart Circ Physiol 286: H1757–H1766, 2004.Link | ISI | Google Scholar6. DiFrancesco D. A new interpretation of the pace-maker current in calf Purkinje fibres. J Physiol 314: 359–376, 1981.Crossref | PubMed | ISI | Google Scholar7. DiFrancesco D. The cardiac hyperpolarizing-activated current, If. Origins and developments. Prog Biophys Mol Biol 46: 163–183, 1985.Crossref | PubMed | ISI | Google Scholar8. DiFrancesco D. The role of the funny current in pacemaker activity. Circ Res 106: 434–446, 2010.Crossref | PubMed | ISI | Google Scholar9. DiFrancesco D, Camm JA. Heart rate lowering by specific and selective If current inhibition with ivabradine: a new therapeutic perspective in cardiovascular disease. Drugs 64: 1757–1765, 2004.Crossref | PubMed | ISI | Google Scholar10. DiFrancesco D, Ojeda C. Properties of the current if in the sino-atrial node of the rabbit compared with those of the current IK, in Purkinje fibres. J Physiol 308: 353–367, 1980.Crossref | PubMed | ISI | Google Scholar11. Guo J, Ono K, Noma A. A sustained inward current activated at the diastolic potential range in rabbit sino-atrial node cells. J Physiol 483: 1–13, 1995.Crossref | PubMed | ISI | Google Scholar12. Himeno Y, Toyoda F, Satoh H, Amano A, Cha CY, Matsuura H, Noma A. Minor contribution of cytosolic Ca2+ transients to the pacemaker rhythm in guinea pig sinoatrial node cells. Am J Physiol Heart Circ Physiol (10 15, 2010). doi: https://doi.org/10.1152/ajpheart.00764.2010.PubMed | ISI | Google Scholar13. Ju YK, Allen DG. Intracellular calcium and Na+-Ca2+ exchange current in isolated toad pacemaker cells. J Physiol 508: 153–166, 1998.Crossref | PubMed | ISI | Google Scholar14. Kurata Y, Hisatome I, Imanishi S, Shibamoto T. Roles of L-type Ca2+ and delayed-rectifier K+ currents in sinoatrial node pacemaking: insights from stability and bifurcation analyses of a mathematical model. Am J Physiol Heart Circ Physiol 285: H2804–H2819, 2003.Link | ISI | Google Scholar15. Lakatta EG. A paradigm shift for the heart's pacemaker. Heart Rhythm 7: 559–564, 2010.Crossref | PubMed | ISI | Google Scholar16. Lakatta EG, DiFrancesco D. What keeps us ticking: a funny current, a calcium clock, or both? J Mol Cell Cardiol 47: 157–170, 2009.Crossref | PubMed | ISI | Google Scholar17. Lakatta EG, Maltsev VA, Bogdanov KY, Stern MD, Vinogradova TM. Cyclic variation of intracellular calcium: a critical factor for cardiac pacemaker cell dominance. Circ Res 92: 45e–50e, 2003.Crossref | PubMed | ISI | Google Scholar18. Lakatta EG, Vinogradova TM, Maltsev VA. The missing link in the mystery of normal automaticity of cardiac pacemaker cells. Ann NY Acad Sci 1123: 41–57, 2008.Crossref | PubMed | ISI | Google Scholar19. Li J, Qu J, Nathan RD. Ionic basis of ryanodine's negative chronotropic effect on pacemaker cells isolated from the sinoatrial node. Am J Physiol Heart Circ Physiol 273: H2481–H2489, 1997.Link | ISI | Google Scholar20. Mattick P, Parrington J, Odia E, Simpson A, Collins T, Terrar D. Ca2+-stimulated adenylyl cyclase isoform AC1 is preferentially expressed in guinea-pig sino-atrial node cells and modulates the If pacemaker current. J Physiol 582: 1195–1203, 2007.Crossref | PubMed | ISI | Google Scholar21. Mitsuiye T, Shinagawa Y, Noma A. Sustained inward current during pacemaker depolarization in mammalian sinoatrial node cells. Circ Res 87: 88–91, 2000.Crossref | PubMed | ISI | Google Scholar22. Noble D. A modification of the Hodgkin-Huxley equations applicable to Purkinje fibre action and pace-maker potentials. J Physiol 160: 317–352, 1962.Crossref | PubMed | ISI | Google Scholar23. Ono K, Iijima T. Pathophysiological significance of T-type Ca2+ channels: properties and functional roles of T-type Ca2+ channels in cardiac pacemaking. J Pharm Sci 99: 197–204, 2005.Crossref | PubMed | ISI | Google Scholar24. Ono K, Iijima T. Cardiac T-type Ca2+ channels in the heart. J Mol Cell Cardiol 48: 65–70, 2010.Crossref | PubMed | ISI | Google Scholar25. Vassalle M. Cardiovascular controversies: the pacemaker current, If, does not play an important role in regulating Sa node pacemaker activity. Cardiovasc Res 30: 309–310, 1995.Crossref | PubMed | ISI | Google Scholar26. Verkerk AO, van Ginneken AC, Wilders R. Pacemaker activity of the human sinoatrial node: role of the hyperpolarization-activated current, If. Int J Cardiol 132: 318–336, 2009.Crossref | PubMed | ISI | Google Scholar27. Vinogradova TM, Zhou YY, Bogdanov KY, Yang D, Kuschel M, Cheng H, Xiao RP. Sinoatrial node pacemaker activity requires Ca2+/calmodulin-dependent protein kinase II activation. Circ Res 87: 760–767, 2000.Crossref | PubMed | ISI | Google Scholar28. Younes A, Lyashkov AE, Graham D, Sheydina A, Volkova MV, Mitsak M, Vinogradova TM, Lukyanenko YO, Li Y, Ruknudin AM, Boheler KR, van Eyk J, Lakatta EG. Ca2+-stimulated basal adenylyl cyclase activity localization in membrane lipid microdomains of cardiac sinoatrial nodal pacemaker cells. J Biol Chem 283: 14461–14468, 2008.Crossref | PubMed | ISI | Google Scholar29. Zhou Z, Lipsius SL. Na+-Ca2+ exchange current in latent pacemaker cells isolated from cat right atrium. J Physiol 466: 263–285, 1993.PubMed | ISI | Google ScholarAUTHOR NOTESAddress for reprint requests and other correspondence: R. B. Robinson, Columbia Univ. Medical Ctr., Dept. of Pharmacology, 630 W. 168th St., Rm. PH7W-318, New York, NY 10032 (e-mail: email protectededu). Download PDF Previous Back to Top Next FiguresReferencesRelatedInformation Cited ByHCN4 current during human sinoatrial node-like action potentialsProgress in Biophysics and Molecular Biology, Vol. 166A Brief History of Pacemaking22 January 2020 | Frontiers in Physiology, Vol. 10Heart Wall30 November 2012Atrial cardiomyocyte calcium signallingBiochimica et Biophysica Acta (BBA) - Molecular Cell Research, Vol. 1813, No. 5 More from this issue > Volume 300Issue 1January 2011Pages H31-H32 Copyright & PermissionsCopyright © 2011 the American Physiological Societyhttps://doi.org/10.1152/ajpheart.01083.2010PubMed21037230History Published online 1 January 2011 Published in print 1 January 2011 Metrics
Richard B. Robinson (2010) conducted an editorial in Sinoatrial node automaticity. The membrane and Ca2+ clock mechanisms of sinoatrial node automaticity are likely interdependent, contributing distinctly to short-term and long-term regulation of pacemaker rate.