Time-dependent changes of calcium influx and efflux rates in rabbit skeletal muscle sarcoplasmic reticulum

Abstract

Unfractionated and low buoyant density sarcoplasmic reticulum vesicles released calcium spontaneously after ATP- or acetyl phosphate-supported calcium uptake when internal Ca 2+ was stabilized by the use of 50 mM phosphate as calcium-precipitating anion. This spontaneous calcium release could not be attributed to falling Ca 2+ concentration outside the vesicles (Ca 0 2+), substrate depletion, ADP accumulation, nonspecific membrane deterioration or the attainment of a high vesicular calcium content. Instead, spontaneous calcium release was directly proportional to Ca 0 2+ at the time that calcium content was maximal. A causal relationship between high Ca 0 2+ and spontaneous calcium release was suggested by the finding that elevation of Ca 0 2+ from less than 1 μM to 3–5 μM increased the rate and extent of calcium release. The spontaneous calcium release was due both to acceleration of calcium efflux and slowing of calcium influx that was not accompanied by a significant change in the rate of ATP hydrolysis. Neither reversal of the transmembrane KCl gradient nor incubation with cation and proton ionophores abolished the spontaneous calcium release. The persistence of calcium release under conditions where the membrane was permeable to both anions and cations makes it unlikely that this phenomenon is due to a changing transmembrane potential.