Hypoxic hypometabolism in the anesthetized turtle, Trachemys scripta

A hypometabolic response during acute exposure to hypoxia has been measured in both endothermic and ectothermic vertebrates. In the turtle, we determi ned the metabolic response to normocapnic hypoxia and hypercapnic hypoxia. In addition, we tested the hypothesis that hypoxic hypometabolism was a reg ulated response that did not depend on O-2 availability. Metabolic, cardiov ascular, and blood gas measurements were collected in anesthetized turtles under two conditions: during normocapnic hypoxia [fractional inspired O-2 F IO2 = 0.1 and 0.05] and during hypercapnic hypoxia [FIO2 = 0.1 and 0.05 plu s fractional inspired CO2 (FICO2) = 0.05]. During normoxia, rate of O-2 con sumption ((V) over dot (O2)) was 0.82 ml.min(-1). kg(-1) and was reduced by nearly 30% at the lowest FIO2. Normocapnic hypoxia of FIO2 = 0.1 had no si gnificant effect on (V) over dot (O2). The addition of 5% CO2 to the inspir ed air did not enhance the effects of hypoxia. Injections of 2,4-dinitrophe nol increased (V) over dot (O2) during hypercapnic hypoxia in some animals to levels greater than those measured during normoxia. We conclude that hyp oxia produces a hypometabolic state in anesthetized turtles, and the pharma cological stimulation of (V) over dot (O2) counteracts the effects of hypox ia on metabolism. The hypoxic hypometabolism in turtles most likely represe nts a regulated response and does not reflect limited O-2 availability at t he cellular level. Finally, we hypothesize that hypoxemia induced by the ri ght-to-left cardiac shunt often associated with diving may trigger the deve lopment of a hypometabolic state and therefore contribute to the prolongati on of aerobic dive times.