Calcium movements promoted by vesicles in a highly enriched sarcolemma preparation from canine ventricle. Calcium-calcium countertransport

Abstract

Calcium uptake by vesicles in a highly enriched sarcolemma preparation from canine ventricle was found to be markedly stimulated by intravesicular calcium. Stimulation of calcium uptake appeared to be a saturable function of intravesicular calcium. Calcium efflux from the vesicles was stimulated by calcium in the reaction medium. Calcium uptake, supported by intravesicular calcium, and calcium efflux, stimulated by extravesicular calcium, were found to correspond on a one-to-one basis. Only small changes in net uptake or efflux were observed to occur in response to chemical gradients of calcium across the membrane. It was concluded, therefore, that under certain conditions, the major means for calcium movement across vesicles in the preparation is via a one-to-one exchange of calcium. Sodium was found to stimulate calcium uptake when present in the intravesicular space and to stimulate calcium efflux when present in the extravesicular space, but the effects of calcium plus sodium were not additive with respect to stimulating either calcium uptake or efflux. The effects of unlabeled calcium, strontium, barium, and magnesium on calcium uptake stimulated by intravesicular calcium and by intravesicular sodium were similar though not identical. The temperature dependence for calcium-stimulated and sodium-stimulated calcium movements was characterized by Q10 values of 1.27 and 2.06, respectively. Previous work has associated the sodium-calcium exchange reaction with the sarcolemma. It is argued that the present study, in turn, provides evidence that the calcium-calcium exchange reaction is also associated with the sarcolemma. In addition, the results of the study are consistent with the hypothesis that one membrane system can promote the exchange of either calcium for calcium or calcium for sodium.