Rate of diastolic Ca release from the sarcoplasmic reticulum of intact rabbit and rat ventricular myocytes

The sarcoplasmic reticulum (SR) of cardiac myocytes loses Ca during rest. In the present study, we estimated the rest-dependent unidirectional Ca efflux from the SR in intact rabbit and rat ventricular myocytes. We determined the time course of depletion of the SR Ca content (assessed as the amount of Ca released by caffeine) after inhibition of the SR Ca-ATPase by thapsigargin. Before rest intervals in Na-containing, Ca-free solution, a 3- min preperfusion with 0Na,0Ca solution was performed to deplete Na(i) but keep the SR Ca content constant. The decrease in Na(i) should stimulate Ca efflux via Na/Ca exchange when Na(o) is reintroduced. Thapsigargin treatment was limited to the last 2 min of preperfusion with 0Na,0Ca solution to minimize SR Ca loss before addition of Na, while attaining complete block of the SR Ca pump. Total SR Ca content was estimated from the [Ca](i) transient evoked by caffeine, taking into account passive cellular Ca buffering. The time constants for SR Ca loss after thapsigargin were 385 and 355 s, whereas the pre-rest SR Ca content was estimated to be 106 and 114 μM (μmol/l nonmitochondrial cell volume) in rabbit and rat myocytes, respectively. The unidirectional Ca efflux from the SR was similar in the two cell types (rabbit: 0.27 μM s^-1; rat: 0.32 μM s^-1). These values are also comparable with that estimated from elementary Ca release events ('Ca sparks,' 0.2-0.8 μM s^-1). Thus, resting leak of Ca from SR may be primarily via occasional openings of SR Ca release channels. Finally, this flux is very slow compared with other Ca transporters in ventricular myocytes.