Electrochemical Na+ and Ca2+gradients drive coupled-clock regulation of automaticity of isolated rabbit sinoatrial nodal pacemaker cells

Abstract

Coupling of an intracellular Ca2+ clock to surface membrane ion channels, i.e., a “membrane clock,” via coupling of electrochemical Na+ and Ca2+ gradients (ENa and ECa, respectively) has been theorized to regulate sinoatrial nodal cell (SANC) normal automaticity. To test this hypothesis, we measured responses of [Na+] i, [Ca2+] i, membrane potential, action potential cycle length (APCL), and rhythm in rabbit SANCs to Na+ /K+ pump inhibition by the digitalis glycoside, digoxigenin (DG, 10–20 μmol/l). Initial small but significant increases in [Na+] i and [Ca2+] i and reductions in E Na and E Ca in response to DG led to a small reduction in maximum diastolic potential (MDP), significantly enhanced local diastolic Ca2+ releases (LCRs), and reduced the average APCL. As [Na+] i and [Ca2+] i continued to increase at longer times following DG exposure, further significant reductions in MDP, ENa, and ECa occurred; LCRs became significantly reduced, and APCL became progressively and significantly prolonged. This was accompanied by increased APCL variability. We also employed a coupled-clock numerical model to simulate changes in ENa and ECa simultaneously with ion currents not measured experimentally. Numerical modeling predicted that, as the E Na and E Ca monotonically reduced over time in response to DG, ion currents (ICaL, ICaT, If, IKr, and IbNa) monotonically decreased. In parallel with the biphasic APCL, diastolic INCX manifested biphasic changes; initial INCX increase attributable to enhanced LCR ensemble Ca2+ signal was followed by INCX reduction as ENCX (ENCX = 3ENa-2ECa) decreased. Thus SANC automaticity is tightly regulated by ENa, ECa, and ENCX via a complex interplay of numerous key clock components that regulate SANC clock coupling.