The Inductions of Tyrosine Aminotransferase by Glucagon and Hydrocortisone*

Glucagon alone, given to adrenalectomized rats, caused induction of tyrosine aminotransferase only in repeated doses. A single dose given with or 5 hours after hydrocortisone enhanced 3-fold the induction of tyrosine aminotransferase by the hydrocortisone. Glucagon did not alter the levels of tryptophan oxygenase in these situations. The simultaneous administration of actinomycin D completely inhibited the effects of glucagon on tyrosine aminotransferase when glucagon was given alone, with hydrocortisone, or after hydrocortisone. The induction of tyrosine aminotransferase by glucagon both in untreated rats and in rats treated by injection of hydrocortisone therefore appears to require synthesis of ribonucleic acid; this requirement is distinct from the RNA synthesis required in the induction of the same enzyme by hydrocortisone. Actinomycin given with the hydrocortisone prevented its induction of tyrosine aminotransferase and tryptophan oxygenase, but actinomycin given more than 1 hour after hydrocortisone did not decrease or increase the levels of these enzymes.