On the mechanism of stimulation of ureagenesis by gluconeogenic substrates: role of pyruvate carboxylase.

Gluconeogenic substrates, lactate or pyruvate, or ornithine produced 100% increase of urea synthesis from NH4Cl. The combined administration of ornithine and lactate (or pyruvate) produced more than additive effects, indicating that they acted at different steps in a potentiating manner. The uptake of ornithine was enhanced by gluconeogenic substrates. This finding may explain, at least in part, the stimulating effect of these substrates on ureagenesis from NH4Cl and ornithine. The gluconeogenic substrate-induced stimulation of ureagenesis from NH4Cl was still observed under conditions of reduced flux through pyruvate carboxylase, ruling out that their action was exclusively mediated by the anaplerotic effect of this enzyme. Pyruvate was a more potent stimulator of ureagenesis than lactate and its effect less sensitive to pyruvate carboxylase inhibition. These observations indicate that a correlation exists between stimulation of ureagenesis by gluconeogenic substrates and flux through pyruvate dehydrogenase. It is concluded that gluconeogenic substrates may stimulate ureagenesis from NH4Cl by 1) increasing intracellular ornithine availability and/or 2) enhancing flux through pyruvate dehydrogenase and consequently the tricarboxylic acid cycle activity.