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1 Department of Ecology and Evolutionary Biology, University of California, Irvine, California; and 2 Center for Respiratory Adaptations, Odense University, DK-5230 Odense, Denmark
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 = 0.1 and
0.05] and during hypercapnic hypoxia [FIO2 = 0.1 and 0.05 plus fractional inspired CO2
(FICO2) = 0.05].
During normoxia, rate of O2
consumption (
O2) was
0.82 ml · min
1 · kg1 and was reduced by
nearly 30% at the lowest
FIO2. Normocapnic hypoxia of
FIO2 = 0.1 had no
significant effect on O2. The addition of 5% CO2 to
the inspired air did not enhance the effects of hypoxia. Injections of
2,4-dinitrophenol increased O2 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
O2 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.
hypoxia; reptile; metabolism; 2,4-dinitrophenol; cardiac shunts; diving
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