Altered endothelial function in lambs with pulmonary hypertension and acute lung injury

Acute lung injury produces pulmonary hypertension, altered vascular reactivity, and endothelial injury. To determine whether acute lung injury impairs the endothelium-dependent regulation of pulmonary vascular tone, 16 lambs were studied during U46619-induced pulmonary hypertension without acute lung injury, or air embolization-induced pulmonary hypertension with acute lung injury. The hemodynamic responses to endothelium-dependent (acetylcholine, ATP, ET-1, and 4 Ala ET-1 [an ETb receptor agonist]) and endothelium-independent (nitroprusside and isoproterenol) vasodilators were compared. During U46619-induced pulmonary hypertension, all vasodilators decreased pulmonary arterial pressure and vascular resistance (P < 0.05). During air embolization-induced pulmonary hypertension, the pulmonary vasodilating effects of acetylcholine, ATP, and 4 Ala ET-1 were attenuated (P < 0.05); the pulmonary vasodilating effects of nitroprusside and isoproterenol were unchanged; and the pulmonary vasodilating effects of ET-1 were reversed, producing pulmonary vasoconstriction (P < 0.05). During air embolization, the pulmonary vasoconstricting effects of ET-1 were blocked by BQ 123, an ETa receptor antagonist. The systemic effects of the vasoactive drugs were similar during both conditions. We conclude that pulmonary hypertension with acute lung injury induced by air embolization results in endothelial dysfunction; there is selective impairment of endothelium-dependent pulmonary vasodilation and an altered response to ET-1 from pulmonary vasodilation to vasoconstriction. This altered response to ET-1 is associated with decreased ETb receptor-mediated vasodilation and increased ETa receptor-mediated vasoconstriction. Endothelial injury and dysfunction account, in part, for the altered regulation of pulmonary vascular tone during pulmonary hypertension with acute lung injury.