Sarcoplasmic Reticulum Ca ²⁺ Refilling Controls Recovery From Ca ²⁺ -Induced Ca ²⁺ Release Refractoriness in Heart Muscle

In cardiac muscle Ca ²⁺ -induced Ca ²⁺ release (CICR) from the sarcoplasmic reticulum (SR) is initiated by Ca ²⁺ influx via L-type Ca ²⁺ channels. At present, the mechanisms underlying termination of SR Ca ²⁺ 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 ²⁺ release could also be responsible for the termination of CICR. To examine the refractoriness of SR Ca ²⁺ release, we analyzed Na ⁺ -Ca ²⁺ exchange currents reflecting cytosolic Ca ²⁺ signals induced by UV-laser flash-photolysis of caged Ca ²⁺ . 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, β-adrenergic stimulation with isoproterenol-accelerated recovery from refractoriness by ≈2-fold. Application of cyclopiazonic acid at moderate concentrations (<10 μmol/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 ²⁺ refilling mediated by the SR Ca ²⁺ -pump corresponds to the rate-limiting step for recovery from CICR refractoriness. Thus, the Ca ²⁺ sensitivity of CICR appears to be regulated by SR Ca ²⁺ content, possibly resulting from a change in the steady-state Ca ²⁺ sensitivity and in the gating kinetics of the SR Ca ²⁺ release channels (ryanodine receptors). During Ca ²⁺ release, the concomitant reduction in Ca ²⁺ sensitivity of the ryanodine receptors might also underlie Ca ²⁺ spark termination by deactivation.