Abstract 943: L-type Ca2+ Current Synchronizes Local Intracellular Ca2+ Oscillators Which Trigger Rhythmic Action Potentials In Developing Cardiomyocytes
Spontaneous rhythmic local Ca2+ releases (LCRs) occurring during late diastolic depolarization (DD) represent a new important mechanism of cardiac cell automaticity. LCRs are produced by partially synchronized firing of the Ca2+ release channels (RyR) in the form of locally propagating subsarcolemmal Ca2+ wavelets. However, Ca2+ wavelets are “noisy” stochastic events, and how the RyR firing can be further temporarily synchronized to provide coordinated, timely (during late DD) signals for spontaneous rhythm is still unclear. It was speculated that LCR synchronization is caused by synchronous depletion of intracellular Ca2+ stores during action potential via Ca2+-induced Ca2+ release (CICR). However, this idea has not been experimentally tested. Using patch clamp and confocal Ca2+ measurements, we studied LCR synchronization in mouse embryonic stem cell-derived cardiocytes (ESCs) of late developmental stage (>7±9d). In rhythmically, spontaneously beating cells, LCRs were mainly present in the second half of DD (n=8). In contrast, in cells exhibiting slow and/or irregular beating, LCR were randomly distributed between beats (n=10). Rhythmically beating cells were then studied under voltage clamp. Activation of L-type Ca2+ current (ICaL) in these cells (n= 10 cells) induced global Ca2+ release, which was followed (after a delay = average LCR period) by a spontaneous, highly synchronized firing of individual local Ca2+ oscillators of various basal rates in different parts of the cell. Subsequent LCR firing gradually became out of phase, ultimately resulting in irregular net LCR activity. Under voltage clamp each synchronized LCR firing caused a spike of an inward current. The current spikes ceased when LCRs were inhibited by 10 μm ryanodine (n=5). We conclude that in spontaneously beating cardiac cells, such as ESCs, LCR firing is synchronized, at least in part, by regularly occurring activation of ICaL and CICR. Synchronized LCR firing contributes to rhythmicity and periodicity of cardiac cell automaticity via timely activation of electrogenic Ca2+-dependent membrane transports. Thus, tight functional integration of intracellular Ca2+ cycling and membrane-delimited processes is required for robust, rhythmic automaticity of these cells.