Contribution of L-3,4-dihydroxyphenylalanine metabolism to the inhibition of gluconeogenesis in rabbit kidney-cortex tubules.

The circulating L-3,4-dihydroxyphenylalanine, the drug of choice in the therapy of Parkinson's disease (PD), is efficiently extracted by kidney and converted to dopamine, known to control several renal functions. As: (i) in addition to liver, kidney is an important source of glucose in mammals and (ii) the action of this drug on renal gluconeogenesis has not yet been studied, the aim of the present investigation was to estimate the influence of L-3,4-dihydroxyphenylalanine metabolism on glucose formation in isolated kidney-cortex tubules incubated with various gluconeogenic substrates. The data indicate that a rapid intracellular degradation of L-3,4-dihydroxyphenylalanine and tyramine (at 100 and 200 microM concentrations) is accompanied by 25-40% decrease in glucose production from pyruvate, alanine + glycerol + octanoate and dihydroxyacetone due to augmented generation of hydrogen peroxide via monoamine oxidase B, resulting in a decline of glutathione redox state by 40%. Moreover, following inhibition of monoamine oxidase B by deprenyl or substitution of pyruvate by aspartate + glycerol + octanoate both L-3,4-dihydroxyphenylalanine and tyramine affect neither the rate of gluconeogenesis nor glutathione redox state. In view of: (i) L-3,4-dihydroxyphenylalanine- and tyramine-induced changes in intracellular levels of gluconeogenic intermediates, and (ii) a significant decline of phosphoenolpyruvate carboxykinase activity by 500 microM oxidized glutathione, it is likely that L-3,4-dihydroxyphenylalanine- and tyramine-evoked disturbances in the glutathione redox state might diminish flux through phosphoenolpyruvate carboxykinase and in consequence decrease glucose formation in renal tubules, suggesting a new potential side-action of L-3,4-dihydroxyphenylalanine treatment.

Amino-acid-dependent, differential effects of ethanol on glucose production in rabbit kidney-cortex tubules.

AIMS: The effect of ethanol on glucose synthesis in kidney-cortex tubules of control and diabetic rabbits has been investigated. METHODS: Both freshly isolated and grown in primary cultures, kidney-cortex tubules were incubated with alanine or aspartate plus lactate or glycerol plus octanoate in the absence and presence of 100 mmol/l ethanol. RESULTS: In freshly isolated renal tubules incubated in the presence of alanine plus lactate or glycerol plus octanoate, and in tubules grown in primary culture in the medium containing alanine plus lactate plus octanoate alcohol, resulted in about 30% decrease in glucose formation. A diminished glucose production in freshly isolated tubules was accompanied by: (i) a decrease in alanine utilization, (ii) an increase in lactate or glycerol consumptions and (iii) a decline in GSH:GSSG ratio. The ethanol action was not abolished by 4-methylpyrazole, an inhibitor of alcohol dehydrogenase (ADH). In view of ethanol-induced changes in gluconeogenic intermediates it is likely that in the presence of alanine plus glycerol plus octanoate ethanol causes a decline in flux through phosphoenolpyruvate carboxykinase, probably due to either an increase in intracellular content of 2-oxoglutarate, inhibitor of this key gluconeogenic enzyme and/or an enhanced flux through pyruvate kinase, as concluded from an increased lactate formation in the presence of glycerol in the incubation medium. In renal tubules grown in primary cultures in the presence of alanine plus lactate plus octanoate a decrease in GSH:GSSG ratio was accompanied by elevated generation of reactive oxygen species (ROS). Upon replacement of alanine by aspartate ethanol affected neither glucose production, substrate uptake, ROS accumulation nor GSH:GSSG ratio. CONCLUSIONS: In the presence of alanine ethanol-induced decrease in glucose production and elevation of ROS might cause a limited NADPH generation resulting in a decrease in the intracellular GSH:GSSG ratio. On the contrary, aspartate might protect against ROS generation, so intensive gluconeogenesis supports NADPH generation and in consequence high values of the intracellular GSH:GSSG ratio are maintained.