Effects of alpha adrenergic blockade and tissue catecholamine depletion on pulmonary vascular response to hypoxia

The highly reactive pulmonary vascular bed of the neonatal calf was utilized to determine whether the hypoxic pulmonary pressor response is modified by alpha-adrenergic blockade with phenoxybenzamine (Group A) or by tissue catecholamine depletion with reserpine (Group B). In addition, in Group A, the effects of hypoxia on the pulmonary circulation were compared and contrasted with those of l-norepinephrine (alpha-receptor stimulator) and isoproterenol (beta-receptor stimulator). In Group A, changes in pulmonary vascular resistance were calculated from measurements of appropriate pressures and of pulmonary blood flow (electromagnetic flowmeter). The increase in pulmonary vascular resistance produced by hypoxia was not diminished by alpha-adrenergic blockade. However, blockade abolished the pulmonary vasoconstrictor effect of norepinephrine. During hypoxic pulmonary vasoconstriction, the administration of either norepinephrine or isoproterenol lowered the pulmonary vascular resistance both before and after alpha-blockade. While this may be a true vasodepressor effect of these drugs it may also reflect passive changes in the pulmonary vessels secondary to an increased pulmonary blood flow.THE PULMONARY VASCULAR RESPONSE TO HYPOXIA IN THE RESERPINIZED CALVES (GROUP B) WAS TESTED UNDER THREE CIRCUMSTANCES: (1) in the awake animal, (2) in the anesthetized animal prepared in the same way as those in Group A, and (3) during constant flow perfusion of the left lower lobe pulmonary artery. From these studies it was concluded that tissue catecholamine depletion did not diminish the pulmonary vascular response to hypoxia.Thus, neither alpha-adrenergic blockade nor tissue catecholamine depletion prevents the hypoxic pulmonary pressor response. Furthermore, alpha-blockade prevents the pulmonary vasoconstrictor response to norepinephrine but not to hypoxia. Therefore it is concluded that hypoxic pulmonary vasoconstriction is not mediated through adrenergic receptor stimulation or release of endogenous catecholamines.