Termination of cardiac Ca2+ sparks: role of intra-SR [Ca2+], release flux, and intra-SR Ca2+ diffusion.

Ca(2+) release from cardiac sarcoplasmic reticulum (SR) via ryanodine receptors (RyRs) is regulated by dyadic cleft [Ca(2+)] and intra-SR free [Ca(2+)] ([Ca(2+)](SR)). Robust SR Ca(2+) release termination is important for stable excitation-contraction coupling, and partial [Ca(2+)](SR) depletion may contribute to release termination. Here, we investigated the regulation of SR Ca(2+) release termination of spontaneous local SR Ca(2+) release events (Ca(2+) sparks) by [Ca(2+)](SR), release flux, and intra-SR Ca(2+) diffusion. We simultaneously measured Ca(2+) sparks and Ca(2+) blinks (localized elementary [Ca(2+)](SR) depletions) in permeabilized ventricular cardiomyocytes over a wide range of SR Ca(2+) loads and release fluxes. Sparks terminated via a [Ca(2+)](SR)-dependent mechanism at a fixed [Ca(2+)](SR) depletion threshold independent of the initial [Ca(2+)](SR) and release flux. Ca(2+) blink recovery depended mainly on intra-SR Ca(2+) diffusion rather than SR Ca(2+) uptake. Therefore, the large variation in Ca(2+) blink recovery rates at different release sites occurred because of differences in the degree of release site interconnection within the SR network. When SR release flux was greatly reduced, long-lasting release events occurred from well-connected junctions. These junctions could sustain release because local SR Ca(2+) release and [Ca(2+)](SR) refilling reached a balance, preventing [Ca(2+)](SR) from depleting to the termination threshold. Prolonged release events eventually terminated at a steady [Ca(2+)](SR), indicative of a slower, [Ca(2+)](SR)-independent termination mechanism. These results demonstrate that there is high variability in local SR connectivity but that SR Ca(2+) release terminates at a fixed [Ca(2+)](SR) termination threshold. Thus, reliable SR Ca(2+) release termination depends on tight RyR regulation by [Ca(2+)](SR).