Effects of pH on Na-Ca Exchange in Canine Cardiac Sarcolemmal Vesicles
نویسندگان
چکیده
Using highly purified sarcolemmal vesicles isolated from dog ventricles, we examined the effects of pH on Na-Ca exchange. The initial rate of Nai-dependent Ca uptake is a sigmoid function of pH. The Ca uptake is inhibited at pH 6 and stimulated at pH 9 (as compared with uptake at pH 7.4). This dependence on pH suggests that the ionization state of a histidine residue may be important in Na-Ca exchange. The effects of H on Nai-dependent Ca uptake are partially competitive with Ca, although this relationship is complex. Nao-dependent Ca efflux is also sensitive to H and increases monotonically with pH. These effects of pH appear to be due to intrinsic interactions with the Na-Ca exchange system and are not due to an alteration of Na-H exchange or membrane permeability. The effects of pH on vesicular Na-Ca exchange are apparent only at low Ca and Na concentrations. Thus modulation of vesicular Na-Ca exchange by pH is manifest only under ionic conditions which exist intracellularly in intact myocardium. Since the negative inotropy caused by acidosis is thought to reflect a fall in internal pH, these results suggest that alteration of sarcolemmal Ca transport (mediated by Na-Ca exchange) by internal pH may contribute to the regulation of myocardial contractility by pH. Circ Res 50: 287-293, 1982
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