MECHANISM OF SUBSTANCE-P-INDUCED HYPERPOLARIZATION OF PORCINE CORONARY-ARTERY ENDOTHELIAL-CELLS

Substance P (SP) is a potent endothelium-dependent vasodilator, and in porcine coronary arterioles the vasodilatory action of SP appears to be mediated entirely by nitric oxide. We tested the hypothesis that SP induces hyperpolarization in porcine coronary artery endothelial cell s (PCAECs) by activating Ca2+-activated K+ (K-Ca) channels. With a bat h Ca2+ concentration ([Ca2+]) of 1 mM, 10 nM SP elicited an increase i n cytosolic [Ca2+] ([Ca2+](i)) from a baseline of 25+/-4 nM to a peak of 808+/-120 nM, followed by a slowly declining plateau phase, which w as absent in Ca2+-free bath and was abolished by addition of extracell ular lanthanum or nickel. Whole cell current-clamp recordings revealed that the time course of SP-induced [Ca2+](i) increases correlated clo sely with membrane hyperpolarization from an average resting potential of -42+/-2 to a peak of -79+/-2 mV. Under voltage clamp, SP stimulate d whole cell currents with reversal potentials strongly dependent on e xtracellular K+ concentration. In 62% of patches tested, single-channe l recordings revealed an intermediate-conductance K+ channel with acti vation highly correlated with the SP-induced [Ca2+](i) increase. These results suggest that, in PCAECs, SP induces Ca2+ release from stores along with Ca2+ influx which activate a K-Ca channel leading to hyperp olarization. This may increase the driving force for Ca2+ entry and th us modulate endothelium-derived nitric oxide release.