CHANGES IN LUMINAL PH CAUSED BY CALCIUM-RELEASE IN SARCOPLASMIC-RETICULUM VESICLES

Fast (milliseconds) Ca2+ release from sarcoplasmic reticulum is an ess ential step in muscle contraction. To electrically compensate the char ge deficit generated by calcium release, concomitant fluxes of other i ons are required. In this study we investigated the possible participa tion of protons as counterions during calcium release. Triad-enriched sarcoplasmic reticulum vesicles, isolated from rabbit fast skeletal mu scle, were passively loaded with 1 mM CaCl2 and release was induced at pCa = 5.0 and pH = 7.0 in a stopped-flow fluorimeter. Accompanying ch anges in vesicular lumen pH were measured with a trapped fluorescent p H indicator (pyranin). Significant acidification (similar to 0.2 pH un its) of the lumen occurred within the same time scale (t(1/2) = 0.75 s ) as calcium release. Enhancing calcium release with ATP or the ATP an alog 5'-adenylylimidodiphosphate (AMPPNP) produced >20-fold faster aci dification rates. In contrast, when calcium release induced with calci um with or without AMPPNP was blocked by Mg2+, no acidification of the lumen was observed. In all cases, rate constants of luminal acidifica tion corresponded with reported values of calcium release rate constan ts. We conclude that proton fluxes account for part (5-10%) of the nec essary charge compensation during calcium release. The possible releva nce of these findings to the physiology of muscle cells is discussed.