Cerebral enzyme changes and tolerance to anoxia during maturation in the rabbit

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Abstract
Newborn rabbits tolerate 30–35 min of anoxia; adult rabbits withstand anoxia for 3–5 min. To gain more information concerning mechanisms of resistance of the newborn mammal to anoxia as compared with the adult, changes in cerebral succinic dehydrogenase and cytochrome oxidase activities were studied in rabbits varying in age from less than 24 hr to adulthood. Succinic dehydrogenase and cytochrome oxidase activities were very low at birth, but gradually increased until 15–18 days postnatally. Between 15–18 days a "critical period" in the development of these enzymes was reached; adult levels of activity were attained. It is also at this critical period in time that the tolerance of the developing rabbit to anoxia becomes as poor as that of the adult. The increase in oxidative enzyme activity is in agreement with the hypothesis that the metabolism of the mammal is transformed from predominantly anaerobic at birth to aerobic with maturation.