Stimulation of Na/Ca exchange by the beta-adrenergic/protein kinase A pathway in guinea-pig ventricular myocytes at 37 degrees C

We investigated the effect of beta-adrenergic stimulation on Na/Ca exchange in whole-cell patch-clamped guinea-pig ventricular myocytes at 37 degrees C. With ion channel and Na/K pump currents blocked, the Na/Ca exchange curr ent (INa-Ca) was measured selectively as membrane current inhibited by 10 n M nickel (Ni) during a voltage ramp applied between +80 and -120 mV. Isopre naline (1 mu M) caused an increase in both inward and outward current gener ated by the Na/Ca exchange, which was prevented by the beta-adrenoceptor bl ocker propranolol. These data suggest that isoprenaline caused a receptor-m ediated up-regulation of Na/Ca exchange activity. Mimicking beta-adrenocept or activation, either by stimulation of adenylate cyclase with forskolin or by internal dialysis of cells with cyclic AMP (3':5'-cyclic adenosine mono phosphate), also increased INa-Ca. Using fluorescence Ca measurement, an in crease of internal cAMP was shown to increase the rate of transmembrane Ca transport via the Na/Ca exchange. A selective inhibitor of protein kinase A prevented stimulation of Na/Ca exchange by isoprenaline. These data sugges t that the underlying mechanism of stimulation was phosphorylation of the N a/Ca exchange protein by protein kinase A. Isoprenaline did not stimulate I Na-Ca when experiments were carried out at 20 degrees C, in contrast to the findings at 37 degrees C. Modulation of Na/Ca exchange by the beta-adrener gic pathway may have important physiological consequences for intracellular Ca regulation and electrical activity during hormonal stimulation, or duri ng sympathetic nerve stimulation.