ABSTRACT
The EDHF-mediated relaxation induced by acetylcholine requires an increase in the cytosolic Ca2 concentration in the endothelial cells and is associated with hyperpolarization of the smooth muscle cells. The aim of the present work was to investigate whether or not agents inhibiting EDHF-evoked responses in the rat superior mesenteric artery could act by preventing the Ca2 response of endothelial cells. EDHF-dependent relaxation of arteries contracted with prostaglandin F2 and hyperpolarization of smooth muscle cells evoked by acetylcholine were abolished by high KCl or by charybdotoxin plus apamin. The relaxations and hyperpolarizations were inhibited by 4-aminopyridine. Ouabain produced a time-dependent inhibition of both responses. In indo-1 loaded arteries, acetylcholine increased the endothelial Ca2
signal with the same potency as it hyperpolarized the smooth muscle cells. Increases in KCl concentration in the bathing solution depressed endothelium Ca2 response to acetylcholine but the inhibition was markedly lower than that of the hyperpolarization of smooth muscle cells. The endothelial Ca2 signal was insensitive to charybdotoxin plus apamin but was depressed by 4-aminopyridine. 4-Aminopyridine abolished the transient thapsigargin-sensitive increase in cytosolic Ca2 evoked by acetylcholine in Ca2-free solution, suggesting that this blocker could affect the Ca2 release process. Ouabain produced a small transitory drop in the Ca2 response of endothelial cells to acetylcholine. These results show that interaction with the Ca2 release in endothelial cells might be involved in the inhibition of EDHF-mediated responses by 4-aminopyridine but not by blockers of Ca2-activated K channels.
