title: Osmotic changes of sarcoplasmic reticulum vesicles during Ca2+ uptake.
abstract: 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.
mesh_terms: D000818:Animals; D001693:Biological Transport, Active / Q000187:drug effects; D002118:Calcium / Q000378:metabolism*; D000252:Calcium-Transporting ATPases / Q000378:metabolism; D066298:In Vitro Techniques; D008298:Maleates / Q000494:pharmacology; D009132:Muscles / Q000378:metabolism; D009995:Osmosis; D011817:Rabbits; D012519:Sarcoplasmic Reticulum / Q000187:drug effects / Q000378:metabolism*
chemical_list: D008298:Maleates; C030272:maleic acid; D000252:Calcium-Transporting ATPases; D002118:Calcium