A proton gradient controls a calcium-release channel in sarcoplasmic reticulum.

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Abstract
Sarcoplasmic reticulum vesicles from mammalian skeletal muscle were previously shown to develop a proton gradient (alkaline inside) of 0.15-0.5 pH units during active Ca2+ uptake. Dissipation of this gradient by the proton ionophores gramicidin, nigericin and carbonyl cyanide p-trichloromethoxyphenylhydrazone caused a rapid transient tension in skinned rabbit psoas muscle fibers. Increases, but not decreases, in medium pH of .apprxeq. 0.2 units over the range from pH 6.5-7.5 elicited transient tensions. In isolated vesicles, physiological levels of Ca2+ (3.3 .mu.M) inhibited pH-induced Ca2+ release. Dicyclohexylcarbodiimide blocked pH- and ionophore-induced Ca2+ release under conditions in which it could bind to sarcoplasmic reticulum proteins but did not inhibit Ca2+ uptake. A proton gradient generated across sardoplasmic reticulum membranes during Ca2+ uptake apparently maintains a Ca2+ release channel in a closed conformation. Dissipation of this gradient apparently permits the Ca2+ release channel to open. Elevated myoplasmic Ca2+ apparently causes the Ca2+ channel to close, permitting Ca2+ uptake through Ca2+/Mg2+-ATPase to function effectively. The proteolipids of sarcoplasmic reticulum bind dicyclohexylcarbodiimide under conditions in which Ca2+ release is blocked and they were previously shown to have Ca2+ ionophoric activity. The Ca2+-release channel apparently either resides in the proteolipids or is controlled by H+ fluxes through the proteolipids.