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

Abstract

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.