Possible Growth Hormone Control of Liver Glutamine Synthetase Activity in Rats*

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Abstract
Hypophysectomy is followed by a decrease in glutamine synthetase activity in rat liver and kidney homogenates; brain enzyme activity is unchanged. A bovine GH regimen increases enzyme activity in the liver and kidney but not in the brain. Maintenance doses of cortisol given to hypophysectomized rats, either alone or in concert with GH, have no effect on glutamine synthetase activity. Glutamate dehydrogenase activity is decreased (∼20%) 2 months after hypophysectomy but not t o the same extent as glutamine synthetase (∼75%). GH has no effect on glutamate dehydrogenase, while cortisol increases the activity of this enzyme. These results suggest that the GH effect on glutamine synthetase activity is a specific action rather than part of a general stimulation of protein synthesis. Although the glutamine synthetase level in the liver is markedly lowered (∼75%) 2 months after hypophysectomy, its percent distribution among different cell fractions, its Michaelis constant for glutamate and hydroxylamine, and its temperature activity profile all remain unchanged. These findings suggest that the glutamine synthetase activity remaining after hypophysectomy is not due t o the presence of a hormone-unresponsive alternate form of the enzyme. Since glutamine formation would divert amino acid nitrogen from urea formation, these findings can explain how increased gluconeogenesis and decreased urea excretion can occur simultaneously during GH treatment. (Endocrinology106: 268, 1980)