Beat-to-Beat Variation in Periodicity of Local Calcium Releases Contributes to Intrinsic Variations of Spontaneous Cycle Length in Isolated Single Sinoatrial Node Cells

Spontaneous, submembrane local Ca2+ releases (LCRs) generated by the sarcoplasmic reticulum in sinoatrial nodal cells, the cells of the primary cardiac pacemaker, activate inward Na+/Ca2+-exchange current to accelerate the diastolic depolarization rate, and therefore to impact on cycle length. Since LCRs are generated by Ca2+ release channel (i.e. ryanodine receptor) openings, they exhibit a degree of stochastic behavior, manifested as notable cycle-to-cycle variations in the time of their occurrence.Aim:The present study tested whether variation in LCR periodicity contributes to intrinsic (beat-to-beat) cycle length variability in single sinoatrial nodal cells.Methods:We imaged single rabbit sinoatrial nodal cells using a 2D-camera to capture LCRs over the entire cell, and, in selected cells, simultaneously measured action potentials by perforated patch clamp.Results:LCRs begin to occur on the descending part of the action potential-induced whole-cell Ca2+ transient, at about the time of the maximum diastolic potential. Shortly after the maximum diastolic potential (mean 54±7.7 ms, n = 14), the ensemble of waxing LCR activity converts the decay of the global Ca2+ transient into a rise, resulting in a late, whole-cell diastolic Ca2+ elevation, accompanied by a notable acceleration in diastolic depolarization rate. On average, cells (n = 9) generate 13.2±3.7 LCRs per cycle (mean±SEM), varying in size (7.1±4.2 μm) and duration (44.2±27.1 ms), with both size and duration being greater for later-occurring LCRs. While the timing of each LCR occurrence also varies, the LCR period (i.e. the time from the preceding Ca2+ transient peak to an LCR's subsequent occurrence) averaged for all LCRs in a given cycle closely predicts the time of occurrence of the next action potential, i.e. the cycle length.Conclusion:Intrinsic cycle length variability in single sinoatrial nodal cells is linked to beat-to-beat variations in the average period of individual LCRs each cycle.