Nitric Oxide–Independent Relaxations to Acetylcholine and A23187 Involve Different Routes of Heterocellular Communication

Abstract

Abstract —NO- and prostanoid-independent relaxations are generally assumed to be mediated by an endothelium-derived hyperpolarizing factor (EDHF) that has been postulated to be an arachidonic acid metabolite. Recent evidence also suggests that direct heterocellular gap junctional communication (GJC) between endothelium and smooth muscle contributes to NO-independent relaxations. In the present study we have investigated the contribution of phospholipase A 2 (PLA 2 )-linked metabolites and GJC to EDHF-type relaxations in rabbit mesenteric artery. In isolated rings preconstricted with 10 μmol/L phenylephrine in the presence of N G -nitro- l -arginine methyl ester (L-NAME) and indomethacin, acetylcholine (ACh) and the Ca 2+ ionophore A23187 evoked relaxations that were markedly attenuated by the Ca 2+ -dependent PLA 2 inhibitors 2-( p -amylcinnamoyl)amino-4-chlorobenzoic acid (3 μmol/L) and arachidonyl trifluoromethyl ketone (3 μmol/L), but were potentiated by the sulfhydryl agent thimerosal (300 nmol/L). In intact rings, relaxations to ACh were attenuated synergistically by L-NAME and Gap 27 peptide, an inhibitor of GJC, whereas ACh-evoked relaxations of “sandwich” preparations were unaffected by the peptide but were abolished by L-NAME. In both ring and sandwich preparations A23187-induced relaxations were attenuated by inhibition of PLA 2 but were insensitive to L-NAME and Gap 27 peptide. We conclude that EDHF-type relaxations of rabbit mesenteric artery to ACh and A23187 depend on a common pathway that involves activation of PLA 2 . In the case of ACh, relaxation requires transfer of a factor or factors from the endothelium to smooth muscle via gap junctions, whereas A23187 permits release directly into the extracellular space.