Effect of troglitazone (Rezulin) on fructose 2,6-bisphosphate concentration and glucose metabolism in isolated rat hepatocytes.

The effect of troglitazone, an orally effective thiazolidinedione, on lactate- and glucagon-stimulated gluconeogenesis (in the absence of insulin) was examined in hepatocytes isolated from rats under different nutritional states. Hepatocytes obtained from fed or 20-24 hr fasted male Sprague-Dawley rats were incubated in Krebs-Henseleit Bicarbonate buffer (KHBC) (in presence or absence of 10.0 mM glucose) containing 2.0 mM [U-14C]lactate (0.1-0.25 microCi) with or without 10.0 nM glucagon and troglitazone (30.0 microM) or the appropriate vehicle. Aliquots were removed at specified endpoints and assayed for glucose and fructose 2,6-bisphosphate (F-2,6-P2) concentrations. In 20-24 hour starved hepatocytes, troglitazone produced a 26.1% inhibition of lactate-stimulated gluconeogenesis. This inhibitory effect of troglitazone on hepatic gluconeogenesis was further potentiated by incubation of the cells with glucose in vitro. In hepatocytes obtained from fasted rats (and incubated with 10 mM glucose in vitro) troglitazone reduced lactate-and glucagon-stimulated gluconeogenesis by 53% and 56%, respectively. This reduction in hepatic glucose production was associated with 1.06 and 1.04 fold increase in the hepatocyte F-2,6-P2 content. In isolated hepatocytes from fed animals and incubated with 10 mM glucose in vitro, troglitazone (15 and 30 microM) did not have any effect on either lactate- or glucagon-stimulated gluconeogenesis. However, 30 microM troglitazone significantly enhanced (36%) F-2,6-P2 concentrations during lactate-stimulated gluconeogenesis. These findings demonstrate that troglitazone decreases hepatic glucose production through alterations in the activity of one or more gluconeogenic/glycolytic enzymes, depending upon the nutritional state of the animal and the presence or absence of hormonal modulation. All of the effects of troglitazone in the present study were observed in the absence of insulin, suggesting an "insulinomimetic" effect. However, this does not exclude the possibility that troglitazone may also function as an "insulin sensitizer" in hepatic and certain other tissues.