Membrane potential fluctuations resulting from submembrane Ca2+ releases in rabbit sinoatrial nodal cells impart an exponential phase to the late diastolic depolarization that controls their chronotropic state

Stochastic but roughly periodic LCRs (Local subsarcolemmal ryanodine receptor-mediated CaReleases) during the late phase of diastolic depolarization (DD) in rabbit sinoatrial nodal pacemaker cells (SANCs) generate an inward current (INCX) via the Na/Ca exchanger. Although LCR characteristics have been correlated with spontaneous beating, the specific link between LCR characteristics and SANC spontaneous beating rate, ie, impact of LCRs on the fine structure of the DD, have not been explicitly defined. Here we determined how LCRs and resultant INCX impact on the DD fine structure to control the spontaneous SANC firing rate. Membrane potential (Vm) recordings combined with confocal Ca measurements showed that LCRs impart a nonlinear, exponentially rising phase to the DD later part, which exhibited beat-to-beat Vm fluctuations with an amplitude of approximately 2 mV. Maneuvers that altered LCR timing or amplitude of the nonlinear DD (ryanodine, BAPTA, nifedipine or isoproterenol) produced corresponding changes in Vm fluctuations during the nonlinear DD component, and the Vm fluctuation response evoked by these maneuvers was tightly correlated with the concurrent changes in spontaneous beating rate induced by these perturbations. Numerical modeling, using measured LCR characteristics under these perturbations, predicted a family of local INCX that reproduced Vm fluctuations measured experimentally and determined the onset and amplitude of the nonlinear DD component and the beating rate. Thus, beat-to-beat Vm fluctuations during late DD phase reflect the underlying LCR/INCX events, and the ensemble of these events forms the nonlinear DD component that ultimately controls the SANC chronotropic state in tight cooperation with surface membrane ion channels.