Numerical Modeling of Flash-Induced Ca2+ Release in Sa Node Cells (SANC) Supports the “Coupled-Clock” Hypothesis of Cardiac Pacemaker Cell Function

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Our recent experiments in SA node cells using flash-induced Ca2+ release from a caged buffer (NP-EGTA) demonstrated that intracellular Ca2+ regulates normal SANC automaticity on a beat-to-beat basis. It cannot be directly envisioned, however, to what extent the reduction in action potential (AP) cycle length upon acute Ca2+ photo-release is attributable to rapid augmentation of Ca2+ pumping into sarcoplasmic reticulum (SR) followed by local Ca2+ releases (LCRs) vs. a direct effect of the flash-released Ca2+ on sarcolemmal proteins. To gain mechanistic insight into the problem, we numerically modeled the effects of flash-released Ca2+ using the 2009-Maltsev-Lakatta model of rabbit SANC that portrays pacemaker cell function as “coupled-clock” system, i.e. coupled intracellular Ca2+ oscillator of the SR and a cell membrane voltage oscillator. Knowing (from prior studies) Ca2+ “on” and “off” rates of NP-EGTA and experimentally measured AP cycle length increase following incubation with the buffer, by varying [NP-EGTA]i in model simulations we estimated [NP-EGTA]i (∼8 mM) required to reproduce experimental results. With this estimated [NP-EGTA]i, we further estimated the rate of Ca2+ release during flash photolysis (a function of laser intensity) to be 1 μM/ms in order to reproduce experimental results on acute AP cycle length shortening. The model also faithfully reproduced diastolic Ca2+ release increase (assessed experimentally as LCR signal mass) following the flash-photolysis. Other numerical models based on a dominance of the membrane voltage oscillator, including the most recent and advanced Kurata et al. model and “Kyoto” model (developed by the Noma's group), could not reproduce the experimental results with any reasonable [NP-EGTA]i (up to 10 mM) and flash-induced Ca2+ release flux (i.e. laser intensity). Our numerical model simulations support the “coupled-clock” hypothesis of cardiac pacemaker cell function.