The outflux of calcium from vesicles of sarcoplasmic reticulum is controlled by the concentration of ionized calcium in the medium. In a calcium-free medium calcium outflux is at a minimum provided ATP and Mg are present. If they are removed calcium outflux becomes rapid and has a rate constant similar to that of outflux during steady-state 40Ca-45Ca exchange with saturating calcium concentrations in the medium. In order to maintain the low permeability state of the membrane only the presence but not the continued hydrolysis of ATP is necessary.

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