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.

Effect of starvation on gene expression of regulatory enzymes of glycolysis/gluconeogenesis in genetically obese (fa/fa) Zucker rats

“…Gluconeogenesis has been reported to be lower in hepatocytes from obese Zucker rats, 13,14,30,31 in spite of the fact that phosphoenolpyruvate carboxykinase mRNA increases at similar levels than in lean animals. These results agree with the transcriptional activation of this gene reported previously.…”

Autoregulation of hepatic glucose production

“…Exton & Park (2) used a saturating amount of lactate (30 mmol/l), which would have made it dif®cult to detect a small inhibition of gluconeogenesis. Conversely, it is possible that gluconeogenic substrates were limiting in the studies of Sa Ânchez-Gutie Ârrez et al (25) and Ruderman & Herrera (5). In the ®rst instance, the incubation medium provided 2 mmol/l lactate and 0.2 mmol/l pyruvate (25), while in the second, the perfusate supplied 1 mmol/l alanine (5).…”

“…Conversely, it is possible that gluconeogenic substrates were limiting in the studies of Sa Ânchez-Gutie Ârrez et al (25) and Ruderman & Herrera (5). In the ®rst instance, the incubation medium provided 2 mmol/l lactate and 0.2 mmol/l pyruvate (25), while in the second, the perfusate supplied 1 mmol/l alanine (5). The duration of fasting (and thus the mass of glycogen available) may also determine whether suppression of gluconeogenesis occurs during hyperglycemia, or whether suppression of glycogenolysis is the primary response.…”

“…The duration of fasting (and thus the mass of glycogen available) may also determine whether suppression of gluconeogenesis occurs during hyperglycemia, or whether suppression of glycogenolysis is the primary response. Neither Sa Ânchez-Gutie Ârrez et al (25) nor Davidson (12) speci®ed how long the rats had been fasted prior to study. Davidson's data, however, indicate that substantial amounts of glycogen were present in the hepatocytes in the basal state, implying that the rats could have been fasted for no more than a few hours (12).…”

Effects of free fatty acids on hepatic glycogenolysis and gluconeogenesis in conscious dogs

“…Data from in vitro studies are similarly conflicting. A study in isolated rat hepatocytes by Sanchez-Gutierrez et al (31) indicated that hyperglycemia significantly inhibited gluconeogenesis. A study by Davidson (12), on the other hand, suggested that hyperglycemia did not do so.…”