THE ROLE OF NITRIC-OXIDE IN THE RESPONSES OF EQUINE DIGITAL VEINS TO VASODILATOR AND VASOCONSTRICTOR AGENTS

Isolated equine digital veins were examined in vitro to study the importance of the endothelium in the responses to both vasodilator and vasoconstrictor agents and to characterise the endothelial-derived mediators involved. Carbachol (Cch; 1 mu M) and bradykinin (Bk; 1 nM) caused relaxation of U44069-induced tone by 79.5 +/- 0.35 % and 73.7 +/- 4.0 % respectively. Mechanical removal of the endothelium completely prevented relaxant responses to Cch and to Bk showing they were mediated by the endothelium. Treatment of veins with N-G-nitro-L-arginine methyl ester (L-NAME; 30 and 300 mu M) inhibited vasorelaxant responses to both Cch and Bk whereas the cyclooxygenase inhibitor, ibuprofen (10 mu M) had no inhibitory effect. The inhibitory action of L-NAME on the relaxations produced by Cch was partly reversed by L-arginine (3 and 10 mM). Cch-relaxations were potentiated in the presence of super oxide dismutase (15 units/ml) and inhibited by methylene blue (10 mu M). The vasorelaxant effects of ATP (0.01 mu M to 0.1 mM) were not dependent on the presence of the endothelium and the selective P-2y receptor agonist, 2-methylthio-ATP proved to be ineffective as a vasodilatoc Removal of the endothelium did not enhance the vasoconstrictor effects of the alpha(1) adrenoceptor agonist phenylephrine (0.01 mu M to 0.1 mM) and treatment with L-NAME (300 mu M) did not change the vasoconstrictor responses to 5-HT (1 nM to 10 mu M) or the alpha(2) adrenoceptor agonist BHT-920 (1 nM to 1 mu M). These data suggest the L-arginine nitric oxide pathway is of primary importance in mediating endothelium dependent relaxations to Cch and BK in equine digital veins but that basal release of nitric oxide is insufficient, under the conditions used in the present study, to modulate responses to vasoconstrictor agents. No functional evidence has been found for P-2y-receptors, alpha(2)-adrenoceptors or 5-HT receptors on endothelial cells of equine digital veins linked to nitric oxide production.