Effects of Glucocorticosteroids and Insulin on Tyrosine Aminotransferase Activity in Isolated Chick Embryo Hepatocytes and in Intact Embryos in Ovo

Hepatocytes were isolated from 17-day-old chick embryos. Steroid hormones or insulin, alone or when combined, did not alter tyrosine aminotransferase (TAT) activity in the isolated hepatocytes. However, TAT was stimulated by glucocorticoids in mixed hepatocyte and fibroblast cocultures; hormonal effects were not observed in pure hepatocyte cultures. Administration of hydrocortisone, dexamethasone, or triamcinolone (singly) to chick embryos in ovo resulted in an increase in hepatic TAT activity; insulin injection was without effect on the enzyme. The stimulation of TAT activity evoked by glucocorticosteroids in ovo was abolished by injection of cycloheximide or cordycepin. These observations contrast with reported glucocorticosteroid actions on TAT activity in fetal rat liver. It would appear that the differential regulatory effects on hepatic TAT by glucocorticosteroids are imposed by the distinct nutrient environments of chick embryos and fetal rats.