L-type Ca2+ channels are not involved in coronary endothelial Ca2+ influx mechanism responsible for endothelium-dependent relaxation

Abstract

Effects of L-type Ca2+ channel blockers on intracellular Ca2+ concentration ([Ca2+]i) changes evoked by the stimulations which cause endothelium-dependent relaxation were examined in freshly isolated pig coronary endothelial cells using fura-2 fluorescent analysis. Substance P and bradykinin produced endothelium-dependent relaxations of pig coronary arteries. The relaxations were inhibited significantly but not completely by N(omega)-nitro-L-arginine (L-NNA) or aspirin, suggesting that nitric oxide (NO), prostacyclin (PGI2) and endothelium-derived hyperpolarizing factor (EDHF) were involved in the responses. Both substance P and bradykinin elevated coronary endothelial [Ca2+]i in a biphasic manner: An initial transient increase was observed within a minute, which was followed by the subsequent sustained increase declining with time. In the medium without Ca2+, substance P-induced elevation of [Ca2+]i was markedly reduced. L-type Ca2+ channel blockers (nicardipine, diltiazem and verapamil) did not affect substance P-induced increase in endothelial [Ca2+]i. In consistent with this finding, Bay k 8644 failed to increase [Ca2+]i in partially depolarized endothelial cells. In contrast, substance P-induced elevation of endothelial [Ca2+]i was suppressed in high K+ solutions. These findings indicate that: (1) Substance P and bradykinin relax pig coronary artery via production/release of NO, PGI2 and EDHF from the endothelium; (2) The synthesis and release of these endothelium-derived factors are accompanied by an increase in endothelial [Ca2+]i; (3) Activation of L-type Ca2+ channels is not involved in coronary endothelial elevation of [Ca2+]i responsible for the production/release of these endothelium-derived factors. L-type Ca2+ channel blockers seem to be advantageous in the application for the disorders of coronary circulation with respect to that they do not prevent endothelial function to produce/release of endogenous vasorelaxants.