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PubMed1993

Regulation of liver glycogen synthesis from [14C]glucose and [14C]lactate by portal-arterial glucose difference in the perfused rat liver.

To study effects of the portal-arterial glucose difference on the hepatic glycogenesis, the liver was isolated from fasted rats and was bivascularly perfused. Thirty-five milliliters of Krebs-Ringer buffer (pH 7.4) with 2 mM glucose, 3 mM lactate, 20 ng/ml insulin, and [1-14C]glucose or [U-14C]lactate was recirculated at flow rates of 14 ml/min via the portal vein and 7 ml/min via the hepatic artery. Glucose was continuously infused at a rate of 27.75 mumol/min into the portal (P experiment) and the arterial cannula (A experiment), and the portal-arterial glucose gradients were +1.98 and -3.96 mM. Perfusate glucose concentration was not different between the P and A experiments within 20 min. Perfusate lactate level was higher in the P experiment than in the A experiment at 20 min. Incorporation of radioactivity from [14C]glucose into glycogen was higher in the P experiment than in the A experiment (0.245 +/- 0.014%/20 min vs 0.175 +/- 0.022%/20 min, P < 0.01), and not influenced by the addition of insulin. Incorporation of 14C from [14C]lactate into glycogen was not different between the P and A experiments, and was significantly increased with the addition of insulin. This activity, in the presence of insulin, was higher in the P experiment than in the A experiment (0.490 +/- 0.028%/20 min vs 0.406 +/- 0.025%/20 min, P < 0.05). These results suggest that the portal-arterial glucose difference has an important role in the regulation of hepatic glycogenesis from exogenous glucose and gluconeogenesis.

Animals↗
PubMed1985

Effect of fructose 1-phosphate on the activation of liver glycogen synthase.

The activation (dephosphorylation) of glycogen synthase and the inactivation (dephosphorylation) of phosphorylase in rat liver extracts on the administration of fructose were examined. The lag in the conversion of synthase b into a was cancelled, owing to the accumulation of fructose 1-phosphate. A decrease in the rate of dephosphorylation of phosphorylase a was also observed. The latency re-appeared in gel-filtered liver extracts. Similar latency was demonstrated in extracts from glucagon-treated rats. Addition of fructose 1-phosphate to the extract was able to abolish the latency, and the activation of glycogen synthase and the inactivation of phosphorylase occurred simultaneously. Fructose 1-phosphate increased the activity of glycogen synthase b measured in the presence of 0.2-0.4 mM-glucose 6-phosphate. According to kinetic investigations, fructose 1-phosphate increased the affinity of synthase b for its substrate, UDP-glucose. The accumulation of fructose 1-phosphate resulted in glycogen synthesis in the liver by inducing the enzymic activity of glycogen synthase b in the presence of glucose 6-phosphate in vivo and by promoting the activation of glycogen synthase.

Animals↗