Relative Contribution of Local Ca2+ Releases (LCRS) and AP-Induced Ca2+ Transient Decay to Diastolic Depolarization in Rabbit Sa Node Cells

Abstract

Background: Intracellular Ca2+ dynamics contribute to cardiac pacemaker cell function via activation of electrogenic Na+/Ca2+ exchanger during diastolic depolarization. Two fundamental mechanisms have been implicated for the diastolic Ca2+ signaling: LCRs and the decaying AP-induced Ca2+ transients. Their relative contributions, however, are unknown.Methods: We evaluated the relative contributions based on simultaneous high-speed camera recordings of Ca2+ signal, alongside membrane potential recordings by perforated patch clamp in single isolated rabbit SA node cells prior to and during beta-adrenergic receptor stimulation with isoproterenol (100 nM). We obtained further insights by employing simulations of our recent numerical model featuring local Ca2+ dynamics in 3D in these cells.Results: All cells tested (n=5) exhibited both LCRs and AP-induced-transient residuals during diastolic depolarization. Individual LCRs have been characterized in the 2D movies using our novel computer algorithm (see our poster on automated analysis of LCRs). The values of LCR signal mass and Ca2+ transient residuals were integrated within the entire cell perimeter in the focal plane of video-recording and compared. Both our analysis of the experimental data and model simulations demonstrated that LCRs provide a major part of the net Ca2+ diastolic signal, both prior to and during isoproterenol exposure. The relative contribution of LCRs, however, substantially increased after isoproterenol application. Specifically, while the contribution of the transient residuals into the net diastolic Ca2+ signal prior to isoproterenol is about 30%, the net signal following isoproterenol becomes almost completely driven by the LCRs, Ca2+ transient decays are accelerated and almost fully complete prior the maximum diastolic depolarization.Conclusions: Both LCRs and local Ca2+ transient residuals contribute to diastolic depolarization. As pacemaker rate increases LCR signal waxes, but the contribution of Ca2+ transient residuals wanes.