Effects of Glucocorticosteroids and Glucagon on Argininosuccinate Synthetase, Argininosuccinase, and Arginase in Fetal Rat Liver*

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Abstract
In the present study we examined the in vivo effects of glucocorticosteroids and glucagon on the developmental formation of the individual urea cycle enzymes argininosuccinate synthetase, argininosuccinase, and arginase during the late fetal period. In particular, addition of exogenous glucagon caused a rise in argininosuccinase and arginase activities in the livers of rat fetuses at term but not at earlier stages. Glucagon produced a rise in argininosuccinase activity earlier if fetuses were previously treated with cortisol. When fetuses were deprived of corticosteroid (hypophysectomy in utero), glucagon no longer promoted the argininosuccinase activity, indicating that adrenal glucocorticoids are required for normal enhancement of the enzyme activity by glucagon. Dibutyryl cAMP was still effective in hypophysectomized fetuses. Results obtained by injected combinations of inducers indicated that a glucocorticosteroid-glucagon interaction might be involved in the regulation of argininosuccinate synthetase and argininosuccinase. No synergistic action was found on arginase activity in vivo.