Spontaneous calcium release from sarcoplasmic reticulum. General description and effects of calcium

A form of spontaneous calcium release from purified sarcoplasmic reticulum isolated from rabbit skeletal muscle is described. The conditions utilized for eliciting spontaneous release rely on preloading the vesicles with calcium in the presence of phosphate. Under the conditions of assay, spontaneous release begins only after a time delay following depletion of calcium ions from the extravesicular space. Release rates as high as 10-20 mumol/mg . min have been observed, but only a portion of the calcium accumulated is released. Released calcium is reaccumulated, and successive spontaneous releases of smaller amounts of calcium are observed under some conditions. Release occurs as a consequence primarily of an increase in unidirectional Ca2+ efflux and, secondarily, a decrease in unidirectional Ca2+ influx. Unidirectional efflux is enhanced by calcium preloading, enhanced by low (0.01-0.1 microM) and reduced by moderate (1-10 microM) extravesicular free calcium levels. Spontaneous Ca2+ release is favored by much lower free calcium concentrations than Ca2+-induced Ca2+ release. The inhibition of unidirectional efflux by calcium appears to involve active calcium uptake. Release is not mediated by a reversal of the calcium pump. The temperature dependence of the release process is steep, comparable with that of energized Ca2+ uptake. This may reflect a process involved in the gating of a hypothetical calcium channel in the sarcoplasmic reticulum membrane.