Impairment of the modulation by glucose of hepatic gluconeogenesis in the genetically obese (fa/fa) Zucker rat

Abstract

Genetically obese (fa/fa) Zucker rats show oral glucose intolerance, an alteration that has been attributed at least in part to an impaired suppression of hepatic glucose output after the ingestion of glucose. In this work, we studied the influence of different concentrations of glucose (2.5-30 mM) on gluconeogenesis from a mixture of [14C]lactate-pyruvate as well as on fructose 2,6-bisphosphate levels, pyruvate kinase activity, and flux through the reaction catalyzed by 6-phosphofructo-1-kinase, in hepatocytes isolated from fed obese (fa/fa) or lean (Fa/-) rats. In hepatocytes isolated from lean rats, incubation with increasing concentrations of glucose caused a dose-dependent inhibition of gluconeogenesis (5.02 +/- 0.54 and 1.82 +/- 0.33 mumol lactate converted to glucose/g cells.20 min in hepatocytes incubated in the presence of 2.5 and 30 mM glucose, respectively; n = 4 experiments; P < 0.01) together with a significant elevation of the fructose 2,6-bisphosphate content and a stimulation of the flux through 6-phosphofructo-1-kinase reaction. Glucose also provoked a dose-dependent activation of pyruvate kinase in the absence of changes in the cellular concentration of cAMP. In liver cells from obese animals, gluconeogenesis was not significantly modified by raising the glucose concentration in the incubation medium (1.26 +/- 0.11 and 0.83 +/- 0.14 mumol lactate converted to glucose/g cells.20 min in hepatocytes incubated with 2.5 and 30 mM glucose, respectively; n = 4 experiments; P = 0.11) despite significant increases in both fructose 2,6-bisphosphate levels and flux through the 6-phosphofructo-1-kinase reaction. In these cells, pyruvate kinase was only slightly activated by high glucose concentrations. These results indicate that, unlike fructose 2,6-bisphosphate levels and flux through the 6-phosphofructo-1-kinase reaction, hepatic gluconeogenesis is unresponsive to high glucose concentrations in genetically obese (fa/fa) rats.