Osmotic changes of sarcoplasmic reticulum vesicles during Ca2+ uptake.

The ATP-dependent accumulation of Ca2+ by sarcoplasmic reticulum vesicles at 37 degrees reaches a peak after approximately 100 sec. The Ca2+-loading level then declines until a steady-state level is reached which is 20% less than the peak value. This spontaneous release of Ca2+ is enhanced by inclusion of maleate in the Ca2+ uptake medium. Increasing the extravesicular osmolarity by the addition of sucrose to the Ca2+ uptake medium prevents spontaneous Ca2+ release and increases the steady-state Ca2+-loading capacity of sarcoplasmic reticulum vesicles. Swelling of sarcoplasmic reticulum vesicles during Ca2+ uptake in medium containing sucrose is indicated by changes in the light-scattering intensity. These experiments indicate that the capacity of sarcoplasmic reticulum vesicles to accumulate Ca2+ is limited by the osmotic gradient generated by the increase in intravesicular Ca2+. Swelling of sarcoplasmic reticulum vesicles during Ca2+ uptake causes spontaneous Ca2+ release.