Luminal Ca2+ content regulates intracellular Ca2+ release in subepicardial myocytes of intact beating mouse hearts: effect of exogenous buffers

摘要

Ca⁺-induced Ca²⁺ release tightly controls the function of ventricular cardiac myocytes under normal and pathological conditions. Two major factors contributing to the regulation of Ca²⁺ release are the cytosolic free Ca²⁺ concentration and sarcoplasmic reticulum (SR) Ca²⁺ content. We hypothesized that the amount of Ca²⁺ released from the SR during each heart beat strongly defines the refractoriness of Ca²⁺ release. To test this hypothesis, EGTA AM, a high-affinity, slow-association rate Ca²⁺ chelator, was used as a tool to modify luminal SR Ca²⁺ content. An analysis of the cytosolic and luminal SR Ca²⁺ dynamics recorded from the epicardial layer of intact mouse hearts indicated that the presence of EGTA reduced the diastolic SR free Ca²⁺ concentration and fraction of SR Ca²⁺ depletion during each beat. In addition, this maneuver shortened the refractory period and accelerated the restitution of Ca²⁺ release. As a consequence of the accelerated restitution, the frequency dependence of Ca²⁺ alternans was significantly shifted toward higher heart rates, suggesting a role of luminal SR Ca²⁺ in the genesis of this highly arrhythmogenic phenomenon. Thus, intra-SR Ca²⁺ dynamics set the refractoriness and frequency dependence of Ca²⁺ transients in subepicardial ventricular myocytes.