Pulmonary vasoconstrictor responses to graded decreases in precapillary blood P(O₂) in intact-chest cat

The effects of graded changes in pulmonary lobar arterial blood P(O₂) and ventilatory hypoxia were investigated in the intact-chest cat under conditions of controlled lobar blood flow. A reduction in precapillary P(O₂) from systemic arterial levels to below 60 Torr increased lobar arterial pressure. Ventilation with 10% O₂ increased lobar arterial pressure, and responses to ventilatory hypoxia and precapillary hypoxemia were independent but additive. The magnitude of the pressor response to precapillary hypoxemia was similar in experiments in which the lung was autoperfused with right atrial blood or cross-perfused with aortic blood from a donor cat breathing 10% O₂. During retrograde perfusion of the ventilated lung, a reduction in pulmonary venous P(O₂) to 40 Torr did not affect inflow pressure. The present data suggest that sensor sites upstream to the alveolar-capillary region in segments of lobar artery unexposed to alveolar gas sense a reduction in precapillary blood P(O₂) and elicit a pulmonary vasoconstrictor response. The sensor site in the precapillary segment is independent of sensors in the alveolar-capillary-exposed segment region, and the effects of stimulation of both sensors on the pulmonary vascular bed are additive. In addition, the present data indicate that sensors in the pulmonary veins do not sense a reduction in P(O₂) in venous blood and elicit a vasoconstrictor response. These data suggest that the mixed venous blood P(O₂) may exert an important regulatory role in controlling pulmonary arterial pressure and pulmonary vascular resistance in the cat under normal and pathological conditions.