Glucose and Insulin Reverse the Effects of Fasting on 3,5,3'-Triiodothyronine Neogenesis in Primary Cultures of Rat Hepatocytes*

The cellular mechanisms by which carbohydrate refeeding reverses the effect of fasting on T3 metabolism were studied in primary cultures of hepatocytes (24 h) harvested from 48-h fasted rats. Net T3 neogenesis (T3 generated from T4) in the fasted hepatocyte preparations (9.2 +/- 0.9 pmol/min X 100 mg protein) was significantly less (P less than 0.001) than that in hepatocyte cultures derived from 72-h glucose-fed rats (41 +/- 0.8 pmol/min X 100 mg protein). Preincubation (18 h) with either glucose (2.5-10 mM) or insulin (10-500 nM) significantly increased the fasted hepatocyte T3 levels to 28 +/- 0.6 and 22 +/- 1.3 pmol/min X 100 mg protein, respectively. Furthermore, incubation with both of these agents demonstrated a greater effect on hepatic T3 neogenesis than with either alone. Fasted hepatocyte T3 neogenesis was enhanced by enrichment with dithiothreitol (5 mM), but the T3 generation remained significantly less than that in cells exposed to glucose or insulin. Studies with glucose analogs demonstrated that preincubation with 2-deoxyglucose (5 mM) significantly increased (P less than 0.001) hepatocyte T3 neogenesis, but 3-O-methylglucose (5 mM) had no effect. In contrast, the insulin-mimetic compounds Concanavalin-A or spermine did not stimulate T3 neogenesis in the fasted hepatocyte cultures. Thus, rat hepatocytes sustained in primary culture for 24 h retain the T3 metabolic characteristics of the intact animal. Glucose and insulin reverse the effect of fasting on hepatocyte T3 neogenesis. The additive response to glucose and insulin suggests that T3 neogenesis is modulated through different mechanisms. The replication of the glucose effect by 2-deoxyglucose and the inability of dithiothreitol to reverse the effect of fasting on hepatocyte T4 5'-deiodinase activity suggest that neither intermediates in the glycolytic pathway nor thiol cofactors mediate the glucose effect. Thus, the restoration of liver T3 metabolism consequent to carbohydrate refeeding of the fasted rat may be mediated by the glucose and insulin responses.