Pulmonary Vasoregulation by Cyclooxygenase Metabolites and Angiotensin II after Hypoperfusion in Conscious, Pentobarbitalanesthetized, and Halothane-anesthetized Dogs

Abstract

The authors investigated the extent to which endogenously produced metabolities of the cyclooxygenase pathway and angiotensin II modulate the pulmonary vascular response to increasing pulmonary blood flow after a period of systemic and pulmonary hypotension and hypoperfusion (defined as posthypoperfusion) in conscious, pentobarbital-anesthetized, and halothane-anesthetized dogs. The authors tested the hypothesis that vasodilator metabolites of the cyclooxygenase pathway offset the vasoconstrictor influence of angiotensin II to prevent pulmonary vasoconstriction posthypoperfusion. Baseline and posthypoperfusion pulmonary vascular pressure-cardiac index (P/Q) plots were constructed by stepwise inflation and deflation, respectively, of a hydraulic occluder implanted around the inferior vena cava to vary Q. In intact (no drug), conscious dogs, the pulmonary vascular P/Q relationship posthypoperfusion was not altered significantly compared with baseline. In contrast, after cyclooxygenase inhibition, active flow-independent pulmonary vasoconstriction (12-17%; P less than 0.01) was observed posthypoperfusion, and this response was abolished entirely by angiotensin converting-enzyme inhibition. During pentobarbital anesthesia, significant pulmonary vasoconstriction (27%; P less than 0.01) occurred posthypoperfusion in the no-drug condition. However, the magnitude of the posthypoperfusion vasoconstriction was not increased by cyclooxygenase inhibition, nor was it reduced by converting-enzyme inhibition. During halothane anesthesia, pulmonary vasoconstriction was not observed posthypoperfusion in the no-drug condition, but it was unmasked (8-13%; P less than 0.05) by cyclooxygenase inhibition and attenuated partially by converting-enzyme inhibition. These results indicate that cyclooxygenase metabolites and angiotensin II exert opposing vasodilator and vasoconstrictor effects, respectively, on the pulmonary circulation of conscious dogs posthypoperfusion. These competing mechanisms are active during halothane anesthesia but are abolished during pentobarbital anesthesia.