Modulation of relaxation to levcromakalim by S-nitroso-N-acetylpenicillamine (SNAP) and 8-bromo cyclic GMP in the rat isolated mesenteric artery

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Abstract
1. Levcromakalim caused concentration-dependent relaxations of methoxamine-induced tone in both endothelium-denuded and intact vessels. Its potency was reduced by the nitric oxide donor, S-nitroso-N-acetylpenicillamine (SNAP; 0.1 microM or 1 microM) in both denuded and intact vessels. The maximal relaxation (Rmax) was reduced only in denuded vessels. 2. SNAP was more potent in endothelium-denuded than intact vessels but there were no differences in Rmax. Glibenclamide (10 microM) did not affect relaxation to SNAP in endothelium-denuded or intact vessels. 3. The soluble guanylyl cyclase inhibitor, 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ, 10 microM) increased the potency and Rmax of levcromakalim in endothelium-intact vessels. ODQ had no effect in denuded vessels. 4. ODQ (10 microM) reduced the vasorelaxant potency of SNAP in both intact and endothelium-denuded vessels by 190-fold and 620-fold, respectively. 5. 8-bromo cyclic GMP (10 or 30 microM) reduced both the potency and Rmax of levcromakalim in de-endothelialized vessels, but had no effect in intact vessels although it reduced both the potency and Rmax of levcromakalim in intact vessels incubated with ODQ (10 microM). 6. In the presence of ODQ (10 microM), SNAP (0.1 microM or 1 microM) reduced the potency of levcromakalim in intact vessels, without altering Rmax, but had no effect in denuded vessels. SNAP (50 microM) reduced both the potency and Rmax of levcromakalim in intact and endothelium-denuded vessels. 7. Therefore, although SNAP causes relaxation principally through generation of cyclic GMP, it can modulate the actions of levcromakalim through mechanisms both dependent on, and independent of, cyclic GMP; the former predominate in endothelium-denuded vessels and the latter in intact vessels.

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