AMINO-ACID-DEPENDENT, DIFFERENTIAL EFFECTS OF ETHANOL ON GLUCOSE PRODUCTION IN RABBIT KIDNEY-CORTEX TUBULES

Abstract

The effect of ethanol on glucose synthesis in kidney-cortex tubules of control and diabetic rabbits has been investigated. 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. 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. 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.