Effects of Glucagon on Glutamate Metabolism in the Perfused Rat Liver

Abstract

Abstract Early, rapid effects of glucagon on glutamate metabolism were studied in rat livers perfused with bicarbonate buffer containing bovine albumin and red cells, [14C]glutamate in tracer amounts, and physiological levels of lactate and pyruvate. Glucagon increased [14C]glucose synthesis from [14C]glutamate to a greater extent than from other labeled substrates. The effect was maximal at 3 min and was not inhibited by fluoroacetate. The hormone decreased the tissue concentrations of glutamate, α-ketoglutarate, and citrate and increased those of alanine, aspartate, and P-pyruvate. In livers perfused with [14C]glutamate for 2½ min, glucagon increased the radioactivities of succinate, fumarate, aspartate, P-pyruvate, alanine, and glutamine, and decreased those of glutamate and α-ketoglutarate. Under these conditions, the specific radioactivities of aspartate, P-pyruvate, and alanine increased. These results are consistent with glucagon stimulation of the conversion of α-ketoglutarate to succinate, of the synthesis of P-pyruvate, and of the transamination of oxalacetate to aspartate and of pyruvate to alanine. Tryptophan markedly inhibited the labeling of P-pyruvate and glucose in livers perfused with [14C]glutamate and increased the tissue levels of intermediates prior to P-pyruvate in the gluconeogenic pathway. Glucagon stimulation of [14C]glucose synthesis was abolished, but the hormone still produced changes in the levels and radioactivities of intermediates consistent with activation of a step between α-ketoglutarate and succinate. Arsenite produced changes in metabolite levels consistent with inhibition of α-ketoglutarate and pyruvate oxidation and abolished the effects of glucagon on [14C]glucose formation and on the levels and radioactivities of intermediates. Glucagon stimulated the release of 14CO2 from [1-14C]-glutamate. The effect was apparently unaltered by fluoroacetate but was abolished by arsenate. Tryptophan diminished the production of 14CO2 in control livers slightly but did not alter the stimulation by glucagon. These data provide further evidence of an effect of glucagon on α-ketoglutarate conversion to succinate and indicate that the effect is not dependent upon the stimulation of gluconeogenesis at another site. In liver perfused with [14C]glutamate for 10 min, glucagon decreased the radioactivities of glutamate, ketoglutarate, malate, citrate, oxalacetate, and pyruvate and increased those of aspartate and P-pyruvate. The specific radioactivities of glutamate, α-ketoglutarate, malate, citrate, and aspartate were reduced by 50% or more. These data indicate that glucagon increases the production of unlabeled glutamate (presumably from endogenous protein); they also provide further evidence for a stimulation of P-pyruvate synthesis from oxalacetate.