The aim of this study was to understand by which intrahepatic mechanism metformin (Met) may inhibit basal hepatic glucose production (HGP) in type 2 diabetes. We studied rats that were fed for 6 weeks a high-fat (HF) diet, supplemented (HF-Met) or not (HF) with Met (50 mg ⅐ kg ؊1 ⅐ day ؊1 ). Basal HGP, assessed by 3-[ 3 H]glucose tracer dilution, was lower by 20% in HFMet rats compared with HF-rats: 41.6 ؎ 0.7 vs. 52 ؎ 1.5 mol ⅐ kg ؊1 ⅐ min ؊1 (means ؎ SE, n ‫؍‬ 5; P < 0.01). Glucose-6 phosphatase (Glc6Pase) activity, assayed in a liver lobe freeze-clamped in situ, was lower by 25% in HF-Met rats compared with HF-rats (7.9 ؎ 0.4 vs. 10.3 ؎ 0.9 mol ⅐ min ؊1 ⅐ g ؊1 wet liver; P < 0.05). Glucose-6 phosphate and glycogen contents, e.g., 42 ؎ 5 nmol/g and 3.9 ؎ 2.4 mg/g, respectively, in HF-rats were dramatically increased by three to five times in HF-Met rats, e.g., 118 ؎ 12 nmol/g and 19.6 ؎ 4.6 mg/g (P < 0.05 and P < 0.01, respectively). Glucose-6 phosphate dehydrogenase activity was increased in HF-Met compared with HF rats (1.51 ؎ 0.1 vs. 1.06 ؎ 0.08 mol ⅐ min ؊1 ⅐ g ؊1 ; P < 0.01). Intrahepatic lactate concentration tended to be lower in the Met-group (؊30%; NS), whereas plasma lactate concentration was higher in HF-Met rats (1.59 ؎ 0.15 mmol/l) than in HF rats (1.06 ؎ 0.06 mmol/l; P < 0.05). We concluded that Met decreases HGP in insulin-resistant HF-fed rats mainly by an inhibition of hepatic Glc6Pase activity, promoting glycogen sparing. Additional mechanisms might involve the diversion of glucose-6 phosphate into the pentose phosphate pathway and an inhibition of hepatic lactate uptake. Diabetes 51:139 -143, 2002

We investigated the intrahepatic mechanisms by which insulin, associated or not with hyperglycemia, may inhibit hepatic glucose production (HGP) in the rat. After a hyperinsulinemic euglycemic clamp in postabsorptive (PA) anesthetized rats, the 70% inhibition of HGP could be explained by a dramatic decrease in the glucose 6-phosphate (G-6-P) concentration, whereas the glucose-6-phosphatase (G-6-Pase) and glucokinase (GK) activities were unchanged. Under hyperinsulinemic hyperglycemic condition, the GK flux was increased. The G-6-P concentration was not or only weakly decreased. The inhibition of HGP involved a significant 25% inhibition of the G-6-Pase activity. Under similar conditions in fasted rats, the GK flux was very low. The suppression of G-6-Pase and HGP did not occur, despite plasma insulin and glucose concentrations similar to those in PA rats. Therefore, 1) insulin suppresses HGP in euglycemia by solely decreasing the G-6-P concentration; 2) when combining both hyperinsulinemia and hyperglycemia, the suppression of HGP involves the inhibition of the G-6-Pase activity; and 3) a sustained glucose-phosphorylation flux might be a crucial determinant in the inhibition of G-6-Pase and of HGP.

Glucagon affects liver glucose metabolism mainly by activating glycogen breakdown and by inhibiting pyruvate kinase, whereas a possible effect on glucose-6-phosphatase has also been suggested. Although such a target is of physiological importance for liver glucose production it was never proven. By using a model of liver cells, perifused with dihydroxyacetone, we show here that the acute stimulation of gluconeogenesis by glucagon (10 ؊7 M) was not related to the significant inhibition of pyruvate kinase but to a dramatic activation of the hydrolysis of glucose 6-phosphate. We failed to find an acute change in glucose-6-phosphatase activity by glucagon, but the increase in glucose 6-phosphate hydrolysis was abolished at 21°C; conversely the effect on pyruvate kinase was not affected by temperature. The activation of glucose 6-phosphate hydrolysis by glucagon was confirmed in vivo, in postabsorptive rats receiving a constant infusion of glucagon, by the combination of a 2-fold increase in hepatic glucose production and a 60% decrease in liver glucose 6-phosphate concentration. Besides the description of a novel effect of glucagon on glucose 6-phosphate hydrolysis by a temperature-sensitive mechanism, this finding could represent an important breakthrough in the understanding of type II diabetes, because glucose 6-phosphate is proposed to be a key molecule in the transcriptional effect of glucose.The metabolic effects of glucagon on liver glucose metabolism have long been described, mediated by both cAMP-and calciumdependent signalings (1-3). Besides a powerful and well documented effect on glycogen breakdown, glucagon also increases liver gluconeogenesis mainly by the allosteric inhibition of pyruvate kinase by its phosphorylation (4, 5). In addition, some effects of glucagon on the fructose phosphate cycle have been described, i.e. inhibition of 6-phosphofructo-1-kinase and activation of fructose-1,6-bisphosphatase (6, 7). Apart from these indisputable effects, a stimulation of glucose-6-phosphatase, a key enzyme in glucose production, has been proposed but never proven (8 -10). In view of a potential key role of glucose 6-phosphate as a cellular intermediate in the transcriptional effects of glucose (11,12), an effect of glucagon on glucose-6-phosphatase could be of major importance. Indeed, despite a prominent effect on liver glucose metabolism, the role of glucagon in the pathogenesis of type II diabetes is still unclear and a matter of debate (13).By studying glucose production under steady state conditions in a model of rat hepatocytes perifused with dihydroxyacetone (DHA) 1 as an exogenous source of carbohydrate, we show in the present work that despite a clear inhibitory effect of glucagon on pyruvate kinase, this effect was not responsible for the enhanced glucose production, indicating that this step is actually not controlling this pathway. Actually, the dramatic increase in the glucose production after glucagon addition was related to a substantial activation of glucose 6-phosphate hydrolysis in glucos...

This study was conducted to test the hypothesis of the activation of glucose-6-phosphatase (G-6-Pase) in situations where the liver is supposed to sustain high glucose supply, such as during the counterregulatory response to hypoglycemia. Hypoglycemia was induced by insulin infusion in anesthetized rats. Despite hyperinsulinemia, endogenous glucose production (EGP), assessed by [3-3H]glucose tracer dilution, was paradoxically not suppressed in hypoglycemic rats. G-6-Pase activity, assayed in a freeze-clamped liver lobe, was increased by 30% in hypoglycemia ( P < 0.01 vs. saline-infused controls). Infusion of epinephrine (1 μg · kg−1 · min−1) in normal rats induced a dramatic 80% increase in EGP and a 60% increase in G-6-Pase activity. In contrast, infusion of dexamethasone had no effect on these parameters. Similar insulin-induced hypoglycemia experiments performed in adrenalectomized rats did not induce any stimulation of G-6-Pase. Infusion of epinephrine in adrenalectomized rats restored a stimulation of G-6-Pase similar to that triggered by hypoglycemia in normal rats. These results strongly suggest that specific activatory mechanisms of G-6-Pase take place and contribute to EGP in situations where the latter is supposed to be sustained.