Regulation by endothelin-1 of Na+-Ca2+ exchange current (INaCa) from guinea-pig isolated ventricular myocytes

The cardiac Na+-Ca2+ exchanger participates in Ca homeostasis, and Na+-Ca2+ exchanger-mediated ionic current (I-NaCa) also contributes to the regulation of cardiac action potential duration. Moreover, I-NaCa can contribute to arrhythmogenesis under conditions of cellular Ca overload. Although it has been shown that the peptide hormone endothelin-1 (ET-1) can phosphorylate the cardiac Na+-Ca2+ exchanger via protein kinase C (PKC), little is known about the effect of ET-1 on I-NaCa. In order to examine the effects of ET-1 on I-NaCa, whole-cell patch clamp measurements were made at 37 degreesC from guinea-pig isolated ventricular myocytes. With major interfering currents inhibited, I-NaCa was measured as the current sensitive to nickel (Ni; 10 mM) during a descending voltage ramp. ET-1 (10 nM) significantly increased I-NaCa (similar to2-fold at - 100 mV). Application of a PKC activator (PMA; 1 muM: phorbol 12-myristate 13-acetate), mimicked the effect of ET-1. In contrast, the PKC inhibitor chelerythrine (CLT, 1 muM) abolished the stimulatory effect of ET-1. An inactive phorbol ester, 4-alpha-phorbol-12,13-didecanoate (4 alpha -PDD, 1 muM) had no effect on I-NaCa. Collectively, these data indicate that ET-1 activated I-NaCa through a PKC-dependent pathway. In additional experiments, isoprenaline (ISO; which has also been reported to activate I-NaCa) was applied. The increase in I-NaCa density with ISO (1 muM) was similar to that induced by ET-1 (10 nM). When I-NaCa was pre-stimulated by ET-1, application of ISO elicited no further increase in current and vice versa. ISO also had no additional effect on I-NaCa when the cells were pretreated with PMA. Application of CLT did not alter the response of I-NaCa to ISO. We conclude that ET-1 stimulated ventricular I-NaCa via a PKC-dependent mechanism under our recording conditions. Concentrations of ET-1 and ISO that stimulated I-NaCa to similar extents when applied separately were not additive when co-applied. The lack of synergy between the stimulatory effects of ET-1 and ISO may be important in protecting the heart from the potentially deleterious consequences of excessive stimulation of I-NaCa. (C) 2001 Harcourt Publishers Ltd.