Synchronization of Local Calcium Releases by Beta-Adrenergic Stimulation in Cardiac Pacemaker Cells

Abstract

Sympathetic heart rate acceleration is a complex orchestration of intercellular, subcellular and molecular events. According to a modern concepts, rhythmic local Ca2+releases (LCRs), generated by a subcellular “Ca-clock”, interact with a “membrane-voltage-clock”, forming a coupled-clock system that produces rhythmic action potentials (AP). We imaged LCRs using a fast-2D-camera with simultaneous AP recording via perforated-patch-clamping to explore how β-AR stimulation modulates the phase and synchronization of the ensemble of individual LCRs within a given diastole to accelerate spontaneous beating rate. In single rabbit SA-node cells (36oC) isoproterenol (100nM): (i) synchronized LCR occurrence [the standard deviation of individual LCR periods (time from Ca2+transient peak to LCR occurrence) decreased (−30%; baseline,71.5ms vs. ISO,50ms)], (ii) shifted the LCR phases to earlier time-point, (iii) shifted “Ca-nadir”, i.e. time-point when AP-induced Ca2+-transient decay ceases and [Ca2+] begins to increase due to the occurrence of LCRs (time from maximum-diastolic-potential to Ca-nadir: baseline, 103ms vs. ISO,86ms); (iv) accelerated diastolic depolarization (baseline,0.09mV/ms vs. ISO,0.15mV/ms); (v) shortened spontaneous cycle length (baseline,420ms vs. ISO,345ms). We previously had demonstrated a close relationship between the decay of the global AP-induced Ca2+ transient and the LCR phase (within a given cycle) and the AP cycle length; we now found that the decays of each LOCAL Ca2+Transient (LCaT) induced by the prior AP vary in different subcellular locations/neighborhoods and that isoproterenol not only accelerated LCaT-decays (average time constant τ: baseline, 104 vs. ISO,87ms) but also synchronized LCaT-decays among neighborhoods (τ's SD/mean: baseline,40.5% vs. ISO,33.7%). More synchronous LCaT-decay mirrors more synchronous local SR Ca2+-pumping and therefore more synchronous achievement of the threshold SR Ca load required for spontaneous LCR occurrence. Thus, earlier and more synchronized LCR occurrence under β-AR stimulation insures an earlier and larger Na+/Ca2+-exchanger current that ultimately accelerates diastolic depolarization rate.