Sarcoplasmic Reticulum Ca Refilling Controls Recovery From Ca -Induced Ca Release Refractoriness in Heart Muscle

In cardiac muscle Ca -induced Ca release (CICR) from the sarcoplasmic reticulum (SR) is initiated by Ca influx via L-type Ca channels. At present, the mechanisms underlying termination of SR Ca release, which are required to ensure stable excitation-contraction coupling cycles, are not precisely known. However, the same mechanism leading to refractoriness of SR Ca release could also be responsible for the termination of CICR. To examine the refractoriness of SR Ca release, we analyzed Na -Ca exchange currents reflecting cytosolic Ca signals induced by UV-laser flash-photolysis of caged Ca . Pairs of UV flashes were applied at various intervals to examine the time course of recovery from CICR refractoriness. In cardiomyocytes isolated from guinea-pigs and mice, -adrenergic stimulation with isoproterenol-accelerated recovery from refractoriness by 2-fold. Application of cyclopiazonic acid at moderate concentrations ( 10 mol/L) slowed down recovery from refractoriness in a dose-dependent manner. Compared with cells from wild-type littermates, those from phospholamban knockout (PLB-KO) mice exhibited almost 5-fold accelerated recovery from refractoriness. Our results suggest that SR Ca refilling mediated by the SR Ca -pump corresponds to the rate-limiting step for recovery from CICR refractoriness. Thus, the Ca sensitivity of CICR appears to be regulated by SR Ca content, possibly resulting from a change in the steady-state Ca sensitivity and in the gating kinetics of the SR Ca release channels (ryanodine receptors). During Ca release, the concomitant reduction in Ca sensitivity of the ryanodine receptors might also underlie Ca spark termination by deactivation. (Circ Res. 2004;95:807-813.)