Steady-state vascular responses to graded hypoxia in isolated lungs of five species

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Abstract
In the isolated pig lung exposed to graded levels of hypoxia, steady-state pulmonary vasomotor tone is maximum at an O2 tension (PO2) of 50 torr. Below 50 torr decreases in PO2 cause steady-state tone to fall below this maximum. To determine whether this stimulus-response relation was peculiar to pigs, the steady-state relation between PO2 and vasomotor tone was measured in the isolated lungs of dogs, rabbits; cats and ferrets, by using indentical techniques in each species. Marked species differences were apparent in the level of PO2 required to elicit responses and the amplitude of the responses. The ferret and the pig had the largest vasoconstrictor responses to hypoxia. No significant responses were obtained in the dog. The cat and rabbit were intermediate responders. In the ferret, cat and rabbit, the stimulus-response relationship was biphasic, as in the pig. On the average, maximal constriction occurred at PO2 of 25 torr. When PO2 was lowered < 25 torr, steady-state tone fell. Pulmonary vasodilation at low PO2 occurs in the isolated lungs of several species.