AREGU July 46/1

Hicks, James W., and Tobias Wang. Hypoxic hypometabolism in the anesthetized turtle, Trachemys scripta. Am. J. Physiol. 277 (Regulatory Integrative Comp. Physiol. 46): R18–R23, 1999.—A hypometabolic response during acute exposure to hypoxia has been measured in both endothermic and ectothermic vertebrates. In the turtle, we determined the metabolic response to normocapnic hypoxia and hypercapnic hypoxia. In addition, we tested the hypothesis that hypoxic hypometabolism was a regulated response that did not depend on O2 availability. Metabolic, cardiovascular, and blood gas measurements were collected in anesthetized turtles under two conditions: during normocapnic hypoxia [fractional inspired O2 FIO2 5 0.1 and 0.05] and during hypercapnic hypoxia [FIO2 5 0.1 and 0.05 plus fractional inspired CO2 (FICO2) 5 0.05]. During normoxia, rate of O2 consumption (V̇O2) was 0.82 ml·min21 · kg21 and was reduced by nearly 30% at the lowest FIO2. Normocapnic hypoxia of FIO2 5 0.1 had no significant effect on V̇O2. The addition of 5% CO2 to the inspired air did not enhance the effects of hypoxia. Injections of 2,4-dinitrophenol increased V̇O2 during hypercapnic hypoxia in some animals to levels greater than those measured during normoxia. We conclude that hypoxia produces a hypometabolic state in anesthetized turtles, and the pharmacological stimulation of V̇O2 counteracts the effects of hypoxia on metabolism. The hypoxic hypometabolism in turtles most likely represents a regulated response and does not reflect limited O2 availability at the cellular level. Finally, we hypothesize that hypoxemia induced by the right-to-left cardiac shunt often associated with diving may trigger the development of a hypometabolic state and therefore contribute to the prolongation of aerobic dive times.