Modification of oxygen tolerance in rats by adaptation to combined hypoxia and hypercapnia.

Tolerance to 100% O2 or to O2 with 60 torr PICO2 (O2-CO2) was determined at pressures of 1.0-4.0 atm abs in normal rats and in rats adapted to combined hypoxia and hypercapnia (HHA) before O2 or O2-CO2 exposure. Results were compared with previous studies of tolerance to O2 or O2-CO2 after adaptation to hypoxia or hypercapnia alone. Both the positive effect on pulmonary O2 tolerance and the negative effect on CNS O2 tolerance found in hypoxia-adapted rats were reduced or eliminated in HHA rats. The increased CNS tolerance to O2-CO2 found in hypercapnia-adapted rats was also reduced in HHA rats. The observation that some of the O2 tolerance modifications associated with adaptation to hypoxia or hypercapnia were reduced or eliminated by adaptation to both stresses concurrently may be because physiologic responses to chronic hypoxia and chronic hypercapnia are opposite in some ways. Results of the present and previous related studies indicate that physiologic adaptations to chronic alterations in the oxygen and acid-base environments have prominent influences on O2 tolerance over a range of useful pressures.