Homogenization of muscle gives a preparation of sealed vesicles derived from the sarcoplasmic reticulum. In the presence of ATP these vesicles will initially accumulate Ca2+ from the external medium and then spontaneously release this Ca2+ in two phases, an initial slow phase and a faster second phase. By measuring ATP concentrations in parallel with measurements of external Ca2+ concentrations we have shown that the second phase of release occurs when the added ATP has been exhausted, but that the first phase of release occurs in the presence of ATP. A similar pattern of uptake and release has been observed in the presence of acetyl phosphate, showing that ADP generated by ATP hydrolysis is not essential for the release process. The temperature-dependence of both phases of release is similar to the temperature-dependence of ATPase activity. Release is dependent on pH over the same pH range as affects binding of Ca2+ to the ATPase. Therefore we propose that Ca2+ release from vesicles of sarcoplasmic reticulum actively loaded with Ca2+ is mediated by the same Ca2+ + mg2+-activated ATPase as is responsible for uptake of Ca2+.

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