Unaltered endothelium-dependent modulation of contraction in the pulmonary artery of hypertensive rats.

Involvement of endothelium-derived nitric oxide (EDNO) in alpha-adrenoceptor agonist-induced contractile responses was studied in isolated pulmonary arteries from Wistar Kyoto rats (WKY) and stroke-prone spontaneously hypertensive rats (SHRSP). In the presence of propranolol, noradrenaline-induced contraction was potentiated by endothelium removal or by N(G)-nitro-L-arginine (L-NOARG). The magnitude of the potentiation was independent of the noradrenaline concentration. L-NOARG also shifted the concentration-response curves for phenylephrine and methoxamine to the left and upward. Contractile responses to 2-amino-5,6,7,8, -tetrahydro-6-ethyl-4H-oxazolo-(5,4-d)-azepine-dihydrochloride (BHT-933) and 5-bromo-6-(2-imidazolin-2-ylamino)-quinoxaline (UK-14304) were augmented by L-NOARG in a concentration-dependent manner. There were no differences in the effects of L-NOARG on the contractile responses to alpha-adrenoceptor agonists between the preparations from WKY and SHRSP. Endothelium-dependent relaxation in response to acetylcholine was not impaired in the preparations from SHRSP when compared with those from WKY. These observations suggest that the contractile responses to the alpha(1)-adrenoceptor agonists were depressed mainly by basally released EDNO, while the responses to the alpha(2)-adrenoceptor agonists were depressed mainly by EDNO released in response to alpha(2)-adrenoceptor stimulation. The comparable influence of the endothelium on the alpha-adrenoceptor agonist-induced contractions in the pulmonary arteries from WKY and SHRSP, which were markedly different from other arteries, could be explained by the unaltered endothelium-dependent relaxation in the preparations from SHRSP.