Histamine receptors in isolated bovine oviductal arteries

The present in vitro study was designed to evaluate the effect of histamine on isolated rings of bovine oviductal artery and to characterize the histamine receptors involved in the histamine-induced response. Endothelial dependence of the response was also investigated. Cumulative addition of histamine and 2-pyridylethylamine (histamine H receptor agonist) induced a concentration-dependent relaxation in intact arterial segments precontracted with noradrenaline. The histamine H1 receptor antagonist mepyramine showed non-competitive antagonism in the histamine-induced concentration-response curve. However, when the response to histamine was evaluated in the presence of mepyramine and histamine H1 and H3 receptors were blocked, Schild analysis yielded a line with a slope of 1.10 and a pA2 value of 8.91, indicating simple competitive antagonism of mepyramine at histamine H1 receptor sites. The histamine H2 receptor agonist, dimaprit, caused marked dilatation only at high doses. Cimetidine, propranolol and mepyramine failed to inhibit this relaxant effect. In precontracted oviductal arteries, cimetidine did not modify the histamine-induced concentration-response curves. Combined treatment with histamine H1 and H2 receptor antagonists did not induce an additional displacement with respect to the isolated effect of mepyramine thus excluding activation of histamine H2 receptors. Histamine and (R)-alpha-methylhistamine, a selective histamine H3 receptor agonist, produced a moderate contractile effect on the resting tone of preparations. Pretreatment with the selective histamine H1 receptor antagonist decreased the (R)-alpha-methylhistamine response but increased the maximal relaxant effect and abolished the contractile effect of histamine, suggesting the presence of a limited population of contractile histamine H3 receptors. Removal of the endothelium or pretreatment with methylene blue produced a significant inhibition of the relaxant response to histamine. Remaining dilatation was practically abolished by mepyramine and also by indomethacin. The L-arginine analogue, N(omega)-nitro-L-arginine methyl ester (L-NAME) inhibited the effect of histamine and basal production of nitric oxide. L-Arginine, which on its own induced significant endothelium-dependent vasodilatation, reversed the effect of L-NAME on histamine relaxation. Indomethacin only caused a slight modification of the sensitivity of the vessels to histamine, suggesting that prostacyclin or other cyclo-oxygenase products did not make a significant contribution to the model. The absence of the endothelium did not modify the contractile effect of histamine. The results suggest that the relaxant response of isolated oviductal arteries to histamine is dependent on the functional integrity of the endothelium and is mainly mediated by histamine H1 receptors. These receptors may mask a minority presence of histamine H3 contractile receptors located on smooth muscle. The main relaxing factor released from the endothelium by mediation of histamine is nitric oxide, which may also exert an effect on vascular tone.