Reduced pulmonary artery vasoconstriction in methacholine in cholesterol-fed rabbits

Alterations in vascular tone are well documented in hypercholesterolemia, yet little is known about the role of dietary cholesterol in endothelium-dependent contractions of pulmonary arteries. Methacholine and arachidonic acid cause endothelium-dependent contractions in normal rabbit pulmonary artery that are mediated by thromboxane A2. We tested the effect of these agonists on pulmonary arteries from rabbits fed standard rabbit chow or chow supplemented with 2% cholesterol for 2 weeks. Arachidonic acid-induced contractions did not differ in the groups. However, methacholine-induced contractions were significantly depressed in cholesterol-fed rabbits. Vascular thromboxane A2 production was similar in normal and cholesterol-fed rabbits. Pretreatment with the nitric oxide synthase inhibitor nitro-L-arginine had no effect on contractions observed with methacholine in normal rabbits but enhanced methacholine-induced contractions in cholesterol-fed rabbits. In norepinephrine-precontracted vessels, methacholine caused a small relaxation response in normal rabbits. In contrast, in cholesterol-fed rabbits, methacholine produced enhanced relaxations, suggesting that cholesterol feeding augments relaxations and decreases contractions by increasing nitric oxide. However, nitric oxide synthase activity in pulmonary arteries from cholesterol-fed and normal rabbits was not different between the two groups. In an additional experiment, the calcium-dependent potassium channel blocker charybdotoxin had little effect on methacholine-induced contractions in cholesterol-fed rabbits. In summary, the present study demonstrates that hypercholesterolemia alters pulmonary artery vascular contractions and relaxations to methacholine. This effect is not mediated by a decreased production of thromboxane A2 or by an increased production of nitric oxide. Although the mechanisms mediating the altered vascular responses is still unknown, the results from this study clearly indicate that the regulation of vascular tone is different in normal and hypercholesterolemic vessels.