Factors causing and reversing vasoconstriction in unventilated lung

Vasoconstriction occuring a unventilated or hypoxic lung was studied in dogs and cats to elucidate mechanisms which both cause and reverse it. Lungs were perfused in vivo at constant pressure or constant blood flow; alternatively blood flow and pressure were measured with minimal operative interference. Stimulus-response curves of lung vessels to hypoxia showed a large response within the physiological range of P02 values. Vasoconstriction in unventilated lung caused by bronchial occlusion sometimes matched that caused by an equal degree of ventilation hypoxia but was sometimes greater. Responses to both stimuli varied widely between animals and in one animal at different times. This could be due to variable availability of a transmitter or variable presence of vasodilator substances. Both histamine and beta-adrenoreceptor stimulants caused pulmonary vasodilatation in unventilated lung. Histamine caused pulmonary vasoconstriction and vasodilatation in different circumstances which could be blocked respectively by H1 and H2 antihistamine drugs. Potent alpha- and beta-adrenoreceptor action on pulmonary vessels was demonstrated in both species. Alpha-adrenoreceptor blocking drugs caused dilatation and beta-adrenoreceptor blocking drugs caused vasoconstriction. The possible role of histamine and catecholamines in causing or reversing hypoxic vasoconstriction or in maintaining pulmonary vascular tone is discussed.