Calcium transport and monovalent cation and proton fluxes in sarcoplasmic reticulum vesicles

ATP-dependent Ca2+ 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 Ca2+ 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 Ca2+ transport, suggesting that active Ca2+ uptake stimulated the release of K+ and H+ from the vesicles. The ratio of K+ (or H+) release to Ca2+ transport was near two. Release of K+ did not appear to be directly catalyzed by the Ca2+-ATPase. Evidence against a directly coupled ATP-mediated 2 K+-Ca2+ or K+-Ca2+ 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) Ca2+ 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 Ca2+ transport system in sarcoplasmic reticulum. The results further suggest that during Ca2+ transport development of a membrane potential (positive inside) is likely nullified by the countermovement of the permeant cations K+, Na+, and H+.