Ornithine-Urea Cycle Activity in Xenopus laevis: Adaptation in Saline

The concentration of urea in the blood and the rate of urea excretion were markedly elevated in Xenopus maintained in hypertonic saline for 2 to 3 weeks. These changes were accompanied by a twofold increase in the activity of the ornithine-urea cycle as measured in liver slices. The activity of carbamoyl phosphate synthetase rose threefold in frogs adapted to saline. These results suggest that changes in activities of urea cycle enzymes may be important in the adaptation of aquatic organisms to environments of varying salinities.

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