In isolated rabbit renal kidney-cortex tubules 2 mM glycerol, which is a poor gluconeogenic substrate, does not induce glucose formation in the presence of alanine, while it activates gluconeogenesis on substitution of alanine by aspartate, glutamate or proline. The addition of either 5 mM 3-hydroxybutyrate or 5 mM acetoacetate to renal tubules incubated with alanine + glycerol causes a marked induction of glucose production associated with inhibition of glutamine synthesis. In contrast, the rate of the latter process is not altered by ketones in the presence of glycerol and either aspartate, glutamine or proline despite the stimulation of glucose formation. Acceleration of gluconeogenesis by ketone bodies in the presence of amino acids and glycerol is probably due to (i) stimulation of pyruvate carboxylase activity, (ii) activation of malate-aspartate shuttle as concluded from elevated intracellular levels of malate, aspartate and glutamate, as well as (iii) diminished supply of ammonium for glutamine synthesis from alanine resulting from a decrease in glutamate dehydrogenase activity.
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