Role of voltage-dependent potassium channels and myo-endothelial gap junctions in 4-aminopyridine-induced inhibition of acetylcholine relaxation in rat carotid artery

Abstract

The present study examined the role of voltage-gated potassium (K v) channels and myo-endothelial gap junctions in 4-aminopyridine-induced inhibition of acetylcholine-evoked endothelium-dependent relaxation and NO release in the rat carotid artery. The acetylcholine-induced relaxation was drastically inhibited by 94% and 82%, respectively in the presence of either 100 µM N G-nitro- l-arginine methyl ester (L-NAME) or 10 µM 1 H-[1,2,4]oxadiazolo[4,3,a]quinoxalin-1-one (ODQ), while it was abolished following endothelium removal. 4-aminopyridine (1 mM), a preferential blocker of the K v channels significantly decreased the vasodilator potency, as well as efficacy of acetylcholine (pD 2 5.7 ± 0.09, R max 86.1 ± 3.5% versus control 6.7 ± 0.10 R max 106 ± 3.5%, n = 6), but had no effect on the relaxations elicited by either sodium nitroprusside (SNP) or 8-bromo-cyclic guanosine monophosphate (8-Br-cGMP). 4-AP (1 mM) also inhibited acetylcholine (3 µM)-stimulated nitrite release in the carotid artery segments (99.4 ± 4.93 pmol/mg tissue weight wt; n = 6 versus control 123.8 ± 7.43 pmol/mg tissue weight wt, n = 6). 18α-glycyrrhetinic acid (18α-GA, 5 µM), a gap junction blocker, completely prevented the inhibition of acetylcholine-induced relaxation, as well as nitrite release by 4-AP. In the pulmonary artery, however antagonism of acetylcholine-evoked relaxation by 4-AP was not reversed by 18α-GA. These results suggest that 4-AP-induced inhibition of endothelium-dependent relaxation and NO release involves electrical coupling between vascular smooth muscle and endothelial cells via myo-endothelial gap junctions in the rat carotid artery, but not in the pulmonary artery. Further, direct activation of 4-AP-sensitive vascular K v channels by endothelium-derived NO is not evident in the carotid blood vessel, while this appears to be an important mechanism of acetylcholine-induced relaxation in the pulmonary artery.