In addition to nitric oxide (NO) and prostacyclin (PGI₂), there is another endothelium-derived mechanism of smooth muscle relaxation, which is associated with an endothelium-derived hyperpolarizing factor (EDHF). To assess the role of gap junctions in endothelium dependent hyperpolarization, we investigated the relationship between distribution of myoendothelial gap junction (MEGJ) and relative importance of the EDHF pathway in the regulation of vascular tone.

Immunohistochemistry and confocal microscopic examination of the mesenteric arterial wall of male Sprague-Dawley rat following treatment with specific antibodies were performed to delineate the distribution of connexin 43, a gap junctional protein. The standard dose-response curve for acetylcholine (10^-9-10^-5 M) of the mesenteric artery was regarded as the release of EDHF in the presence of the NO synthase inhibitor, N^ω-nitro-L-arginine methyl ester (L-NAME, 10^-4 M) and indomethacine (10^-5 M). The effects of the gap junction inhibitors such as 18α-glycyrrhetinic acid (18α-GA; (2 x 10^-4 M)) and carbenoxolone (3 x 10^-4 M) was assessed regarding relaxtion to acetylcholine, contraction to phenylephrine (5 x 10^-6 M) in the proximal and distal mesenteric arteries.

In the distal artery, gap junctional plaques were more prevalent, and the relaxation response to acetylcholine was augmented and the contraction response to phenylephrine was depressed compared with the proximal artery. In both the proximal and distal mesenteric arteries, acetylcholine-induced relaxations attributable to EDHF were near completely blocked by 18α-GA and carbenoxolone to the same degree. Regardless of the presence of L-NAME plus indomethacin, 18α-GA significantly augmented the contraction response to phenylephrine.

The vasomotor regulatory response by EDHF in the rat mesenteric arteries may be explained by extensive heterocellular coupling through MEGJs. Moreover, the release of EDHF through MEGJ may have a essential role in the regulation of resistor arterial tone.