Stimulation of Ca-induced Ca release only transiently increases the systolic Ca transient: Measurements of Ca fluxes and sarcoplasmic reticulum Ca

Objective: To investigate the effects of stimulating calcium induced Ca release with low concentrations (100-200 μM) of caffeine and, in particular, to study the cellular mechanisms responsible for the transient responses found previously. Methods: Experiments were performed on isolated rat ventricular myocytes. Intracellular calcium concentration ([Ca2+](i)) was measured with Indo-1, the cells were voltage-clamped with the perforated patch technique and sarcoplasmic reticulum (s.r.) Ca content was estimated from the integral of the caffeine-evoked current. Results: The systolic Ca transient produced by the first depolarization in the presence of caffeine was larger than the control. Over the next few pulses the magnitude of the Ca transient returned to control levels despite the maintained presence of caffeine. The s.r. Ca content was decreased by 9% after one pulse in caffeine and by 21% after several pulses in caffeine. The first pulse in the low concentration of caffeine was followed by an enhanced inward (Na-Ca exchange) current tail indicating increased efflux of calcium from the cell. The extra loss of calcium calculated from the tail current agreed quantitatively with that from the change of s.r. Ca content. Conclusions: These results show that stimulating calcium induced calcium release produces only a transient increase of the systolic Ca transient. This is due to the larger Ca transient decreasing the s.r. Ca content. It is concluded that any agent whose sole mode of action is stimulation of calcium-induced calcium release will not produce a maintained inotropic effect. The consequences of this for the effects of other modulators of calcium induced calcium release are discussed.