Interacting effects of hypoxia adaptation and acute hypercapnia on oxygen tolerance in rats.

To define the effects of hypoxia adaptation on tolerance to a useful range of O2 pressures, groups of 20 or more rats were exposed to O2 at 1.0, 1.5, 2.0, 3.0, and 4.0 ATA before and after adaptation to an inspired PO2 (PIO2) of 71 Torr for 5 days. Effects of acute hypercapnia on O2 tolerance in hypoxia-adapted rats were also determined by exposing rats to the same O2 pressures with an inspired PCO2 (PICO2) of 60 Torr. In nonadapted rats exposed to O2 at 1.0, 1.5, 2.0, 3.0, and 4.0 ATA, 50% mortality (LD50) occurred at 76.4, 26.8, 17.4, 9.0, and 6.4 h, respectively. LD50 values in O2-CO2 at the same pressures were 77.1, 24.5, 15.6, 3.4, and 1.7 h. Hypoxia-adapted rats had only 20% mortality in O2 at 1.0 ATA, and survivors were killed at 336 h. In O2-CO2 at 1.0 ATA, mortality was 85% with an LD50 at 282 h. LD50 values in hypoxia-adapted rats at O2 pressures of 1.5, 2.0, 3.0, and 4.0 ATA were 63.1, 22.5, 7.9, and 3.8 h, respectively. Corresponding values in O2-CO2 were 29.5, 18.7, 4.9, and 1.9 h. Exposure to O2 at 4.0 ATA caused nearly immediate onset of violent convulsions in hypoxia-adapted rats compared with a 50% incidence of convulsions at 3.2 h in nonadapted rats. These data indicate that hypoxia-adaptation increases pulmonary O2 tolerance but reduces central nervous system (CNS) O2 tolerance. However, the enhanced pulmonary O2 tolerance in hypoxia-adapted rats is greatly diminished when acute hypercapnia is superimposed on O2 exposure.(ABSTRACT TRUNCATED AT 250 WORDS)