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

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, we measured the steady-state relation between PO2 and vasomotor tone in the isolated lungs of dogs, rabbits, cats, and ferrets, by using identical techniques in each species. Marked species differences were apparent in both 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 an PO2 of 25 Torr. When PO2 was lowered below 25 Torr, steady-state tone fell. Thus pulmonary vasodilation at low PO2 occurs in the isolated lungs of several species.