Macro- and micropulmonary hemodynamic changes to nitric oxide blockade in monocrotaline-induced pulmonary hypertension in rats - An in vivo approach

Background and Purpose: Data on the role of nitric oxide (NO) in monocrotaline (MC)-induced pulmonary hypertension of rats have all been derived from in vitro measurements. In vivo pulmonary hemodynamics and microvascular responses to NO blockade in MC-treated rats have not been reported. The current study evaluated the role of NO in live MC-treated rats. Methods: Male Sprague-Dawley rats (n = 29) were divided into saline control and MC-treated groups. Three to 4 weeks after either saline or MC injection, pulmonary hemodynamics were measured using pulmonary arterial catheter and thermodilution techniques. Pulmonary microvascular diameter changes were assessed using an intravital microscope. Results: Three to 4 weeks after subcutaneous injection of MC (50 mg/kg), MC-treated rats showed elevated pulmonary arterial pressure compared to control rats (23.6 ± 2.9 mm Hg vs 14.5 ± 0.8 mm Hg). N^ω-nitro-L-arginine methyl ester (L-NAME, 20 mg/kg, intravenously), an NO synthase inhibitor, produced significant increases in pulmonary arterial pressure and total pulmonary vascular resistance compared to control rats (12.3 ± 2.5 mm Hg vs 1.0 ± 0.9 mm Hg; 0.21 ± 0.04 mm Hg/mL/min vs 0.05 ± 0.02 mm Hg/mL/min, p < 0.05). Intravital microscopic observation of the subpleural vessels showed significant vasoconstriction in medium-sized (> 40 μm) pulmonary arterioles of MC-treated rats following L-NAME administration (58.4 ± 3.6 μm vs 47.1 ± 3.4 μm, p < 0.05) while no significant diameter changes were found in either the small-sized (< 40 μm) pulmonary arterioles or pulmonary venules. Diameters of pulmonary arterioles and venules in control rats were not affected by L-NAME administration. Conclusion: Pulmonary arterial pressure and pulmonary vascular resistance increased after NO blockade in live MC-treated rats. The site of vasoconstriction after NO blockade included medium-sized pulmonary arterioles but not small-sized pulmonary arterioles. This study has provided both macro- and microhemodynamic evidence of a modulatory role of NO in live MC-treated rats.