[Effects of sulfhydryl reagents on sodium-calcium exchange system in canine cardiac sarcolemmal vesicles].

Using a sarcolemmal vesicle-enriched preparation prepared from canine ventricles, the effects of several sulfhydryl reagents on the Na+-Ca2+ exchange were examined. N-Ethylmaleimide (NEM) significantly inhibited the intravesicular Na+ (Na+i)-dependent Ca2+ uptake but did not remarkably affect the extravesicular Na+ (Na+o)-dependent Ca2+ efflux. Iodoacetamide (IAM) had no significant effect on the Na+i-dependent Ca2+ uptake and somewhat promoted the Na+o-dependent Ca2+ efflux. Dithiothreitol (DTT) did not significantly influence the Ca2+ uptake, was inclined to increase passive Ca2+ efflux and apparently decreased the Ca2+ efflux. DTT added with IAM exhibited a significant depressant effect on the Ca2+ uptake and simultaneously had a remarkable increasing effect on passive Ca2+ efflux. NEM and (DTT + IAM), however, did not significantly alter the Km value for Ca2+. p-Chloromercuribenzoate (PCMB) remarkably inhibited the Na+i-dependent Ca2+ uptake but simultaneously showed a remarkable promoting effect on passive Ca2+ efflux, whereby the Na+o-dependent Ca2+ efflux was factually counteracted. These results suggest the possibility that the direction of the Na+-Ca2+ exchange might be controlled by a modification with NEM and IAM of the thiol group(s) in the protein involved in the Na+-Ca2+ counter-transport and that the reductive cleavage of disulfide bond(s) and a modification with PCMB of thiol group(s) present in the sarcolemmal vesicle-associated protein(s) result in an increased Ca2+ permeability in the vesicular membrane.