Calcium transport and monovalent cation and proton fluxes in sarcoplasmic reticulum vesicles.
J. Biol. Chem. 256 (2): 636--643 (January 1981)

ATP-dependent Ca$^2+$ uptake by rabbit skeletal muscle sarcoplasmic reticulum vesicles has been studied in the presence and absence of artificially generated pH gradients and membrane potentials. H$^+$ and K$^+$ diffusion potentials were generated via the H$^+$ and K,Na channels of sarcoplasmic reticulum by transfer of vesicles from low to high pH, or from high to low K$^+$. Membrane potentials were measured using the voltage-sensitive fluorescent dye 3,3'-dipentyl-2,2'-oxacarbocyanine. The initial rate of Ca$^2+$ uptake was found to be increased in the presence of a pH gradient and membrane potential (negative inside). In turn, the rates of decay of K$^+$- or H$^+$-induced membrane potentials were accelerated during Ca$^2+$ transport, suggesting that active Ca$^2+$ uptake stimulated the release of K$^+$ and H$^+$ from the vesicles. The ratio of K$^+$ (or H$^+$) release to Ca$^2+$ transport was near two. Release of K$^+$ did not appear to be directly catalyzed by the Ca$^2+$-ATPase. Evidence against a directly coupled ATP-mediated 2 K$^+$-Ca$^2+$ or K$^+$-Ca$^2+$ exchange reaction was that (i) similar results were obtained when K$^+$ was substituted by Na$^+$ or by organic cations which could rapidly permeate through the channel of K$^+$,Na$^+$-permeable vesicles and (ii) Ca$^2+$ transport did not result in an equivalent release of 86Rb+ or 22Na$^+$ from K$^+$,Na$^+$-impermeable vesicles. These studies are in support of an electrogenic Ca$^2+$ transport system in sarcoplasmic reticulum. The results further suggest that during Ca$^2+$ transport development of a membrane potential (positive inside) is likely nullified by the countermovement of the permeant cations K$^+$, Na$^+$, and H$^+$.
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