Ca spark latency and control of intrinsic Ca release dyssynchrony in rat cardiac ventricular muscle cells.

Cardiac excitation-contraction coupling (ECC) depends on Ca release from intracellular stores via ryanodine receptors (RyRs) triggered by L-type Ca channels (LCCs). Uncertain numbers of RyRs and LCCs form 'couplons' whose activation produces Ca sparks, which summate to form a cell-wide Ca transient that switches on contraction. Voltage (V) changes during the action potential (AP) and stochasticity in channel gating should create variability in Ca spark timing, but Ca transient wavefronts have remarkable uniformity. To examine how this is achieved, we measured the V-dependence of evoked Ca spark probability (P) and latency over a wide voltage range in rat ventricular cells. With depolarising steps, Ca spark latency showed a U-shaped V-dependence, while repolarising steps from 50 mV produced Ca spark latencies that increased monotonically with V. A computer model based on reported channel gating and geometry reproduced our experimental data and revealed a likely RyR:LCC stoichiometry of ∼ 5:1 for the Ca spark initiating complex (IC). Using the experimental AP waveform, the model revealed a high coupling fidelity (P ∼ 0.5) between each LCC opening and IC activation. The presence of ∼ 4 ICs per couplon reduced Ca spark latency and increased P to match experimental data. Variability in AP release timing is less than that seen with voltage steps because the AP overshoot and later repolarization decrease P due to effects on LCC flux and LCC deactivation respectively. This work provides a framework for explaining the V- and time-dependence of P, and indicates how ion channel dispersion in disease can contribute to dyssynchrony in Ca release.