Cellular Metabolite Distribution and the Control of Gluconeogenesis in the Perfused Isolated Rat Liver

Glucose production was measured in isolated rat livers perfused with 100 ml of blood-free recirculating medium. The gluconeogenic rate using L-alanine as substrate was only 55% of that obtained with L-lactate. The steady-state concentration of gluconeogenic and tricarboxylic acid cycle intermediates were measured in freeze clamped biopsies. Livers perfused with L-lactate displayed higher concentrations of malate, alpha-glycerophosphate and beta-hydroxybutyrate probably as a result of a higher state of reduction of the nicotinamide system. Hexose-phosphate intermediates were also increased when L-lactate was the substrate. Phosphoenolpyruvate and 3-phosphoglycerate were considerably elevated when L-alanine was the glucose precursor. Livers perfused with L-lactate displayed higher cytosolic concentration of all the tricarboxylic acid cycle intermediates except oxaloacetate while glutamate was slightly and aspartate considerably higher when alanine was the substrate. In the mitochondrial compartment the pattern of distribution tended to be the opposite; that is, livers perfused with L-lactate showed lower concentrations of all the intermediates except alpha-ketoglutarate. The mitochondrial: cytosolic metabolite gradients of all the intermediates whose distribution was studied were higher in livers perfused with L-alanine. The relevance of these findings to the observed differences in the gluconeogenic fluxes are discussed.