Thyroid hormones are known to stimulate in vivo the synthesis of several liver proteins. In order to determine their role in the regulation of important hepatic proteins at the cell level, the effect of T3 on the induction of the glucoregulatory enzyme phosphoenolpyruvate carboxykinase (PEPck) was investigated in cultured hepatocytes. The cells were isolated from adult hypothyroid rats and exposed to a chemically defined medium, devoid of serum supplement and hormone-free over a period of 48 h.(a) Addition of T3 to the medium had only a minor effect on PEPck induction. (b) Dibutyryladenosine 3',5'-monophosphate (Bt,cAMP) or epinephrine provoked significant increase in PEPck synthesis within 4-6 h. (c) Simultaneous addition of T3 was found significantly to promote the Bt,cAMP-mediated enzyme induction. (d) Physiological concentrations of T3 (10 pM to 1 nM) were found to be effective in enhancing this CAMP-mediated signal. (e) Exposure of the cells to T3 for only 90 min, 16 h prior to addition of Bt,cAMP was nearly as effective as continuous exposure to T3 in enhancing the CAMP-effect on PEPck synthesis. (f) In contrast, no effect of T3 on the Bt,cAMP-induced tyrosine-aminotransferase activity could be detected.It is concluded that the role of T3 on the CAMP-elicited PEPck synthesis is selective, rapid, primarily permissive and significant within the physiological range of the hormone. In addition, these data indicate that such hepatocyte cultures prepared from hypothyroid rats are ideally suited to the analysis of the regulatory role of thyroid hormones on specific gene expression.The synthesis of hepatic PEPck, the rate-limiting enzyme for gluconeogenesis, is under hormonal control by glucocorticoids, glucagon and catecholamines (for a review see [I]). In addition, thyroid hormone injection in vivo significantly enhances PEPck activity [2, 31 as a consequence of a concomitant acceleration in enzyme synthesis [4]. However, thyroid hormone injection is associated with (a) alterations in the response to other hormones (e.g. catecholamines [5]), (b) increases in circulating glucagon [6], and elevation in tissue content of CAMP [6]. In fact, using thc isolated perfused liver the effect of T3 was found to be reduced to only a small increase in the rate of PEPck synthesis [7]. Recent improvements in the use of isolated hepatocytes in culture provide an excellent tool with which to investigate the long-term effects of thyroid hormones on regulating the synthesis of important proteins. We, therefore, investigated the effect of T3 and its interaction with CAMP and epinephrine in the induction of PEPck in isolated hepatocytes from hypothyroid rats incubated in a chemically defined medium.Our results demonstrate a rapid and permissive effect of thyroid hormones at their physiological, circulating concentrations on the epinephrine-induced and CAMP-induced PEPck synthesis.Abbrcviutions. Bt,cAMP, N ',02'-dibutyryladenosine 3',5'-monophosphate; T3, ~-3,3',5-triiodothyronine; PEPck. phosphoenolpyruvate carboxy kinase.Enzymev. Ty...