Effect of dichloroacetate and glucagon on the incorporation of labeled substrates into glucose and on pyruvate dehydrogenase in hepatocytes from fed and starved rats

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Dichloroacetate (2 m m) stimulated the conversion of [1- 14C]lactate to glucose in hepatocytes from fed rats. In hepatocytes from rats starved for 24 h, where the mitochondrial NADH NAD + ratio is elevated, dichloroacetate inhibited the conversion of [1- 14C]lactate to glucose. Dichloroacetate stimulated 14CO 2 production from [1- 14C]lactate in both cases. It also completely activated pyruvate dehydrogenase and increased flux through the enzyme. The addition of β-hydroxybutyrate, which elevates the intramitochondrial NADH NAD + ratio, changed the metabolism of [1- 14C]lactate in hepatocytes from fed rats to a pattern similar to that seen in hepatocytes from starved rats. Thus, the effect of dichloroacetate on labeled glucose synthesis from lactate appears to depend on the mitochondrial oxidation-reduction state of the hepatocytes. Glucagon (10 n m) stimulated labeled glucose synthesis from lactate or alanine in hepatocytes from both fed and starved rats and in the absence or presence of dichloroacetate. The hormone had no effect on pyruvate dehydrogenase activity whether or not the enzyme had been activated by dichloroacetate. Thus, it appears that pyruvate dehydrogenase is not involved in the hormonal regulation of gluconeogenesis. Glucagon inhibited the incorporation of 10 m m [1- 14C]pyruvate into glucose in hepatocytes from starved rats. This inhibition has been attributed to an inhibition of pyruvate dehydrogenase by the hormone (Zahlten et al., 1973, Proc. Nat. Acad. Sci. USA 70, 3213–3218). However, dichloroacetate did not prevent the inhibition of glucose synthesis. Nor did glucagon alter the activity of pyruvate dehydrogenase in homogenates of cells that had been incubated with 10 m m pyruvate in the absence or presence of dichloroacetate. Thus, the inhibition by glucagon of pyruvate gluconeogenesis does not appear to be due to an inhibition of pyruvate dehydrogenase.