Glucose phosphorylation is not rate limiting for accumulation of glycogen from glucose in perfused livers from fasted rats

Effect of glucose on uptake of radiolabeled glucose, 2-DG, and 3-O-MG by the perfused rat liver

In the transition from the fasting to the fed state, plasma glucose levels rise, and the liver converts from an organ producing glucose to one of storage. To determine the effect of glucose on hepatic glucose uptake, radiolabeled glucose, 2-deoxyglucose, and 3-O-methylglucose were injected into perfused rat livers during different nontracer glucose levels, and the concentrations in the outflow were measured. A mathematical model was developed that described the behavior of the injected compounds as they traveled through the liver and was used to simulate and fit the experimental results. The rates of membrane transport, glucokinase, glucose-6-phosphatase, and the consumption of glucose 6-phosphate were estimated. Membrane transport for all of the tracers decreased as nontracer glucose increased, demonstrating competitive inhibition of the glucose transporter. In contrast, the consumption of injected [2-14C]glucose increased when glucose was elevated, demonstrating that glucose caused an activation of enzyme activity that overcame the competitive inhibition of transport and phosphorylation. When glucose was elevated, the rate coefficient of glucokinase did not decrease, indicating that glucokinase was stimulated by glucose. Both changes would lead to the increased glycogen synthesis and decreased glucose production rate observed in vivo during the fasted-to-fed transition.

Effect of hepatectomy on glucose metabolism in the dog

The response of whole-body glucose uptake and oxidation and hindlimb glucose and lactate balance was investigated using the hyperinsulinemic, euglycemic clamp in 12 sham-operated control dogs and 10 hepatectomized dogs. A combination of radioactive (NaH14CO3) and stable (U-13C-glucose) isotope tracers was used to quantify glucose kinetics and oxidation. The insulin concentration was increased to approximately 5,000 microU/mL. Mean glucose uptake rates across the hindlimb were similar, 7.72 and 8.06 mumol.kg-1.min-1 for hepatectomy and sham-operated groups, respectively. Lactate release across the hindlimb also showed no significant differences between the two groups. Therefore, it was concluded that the liver did not affect peripheral glucose uptake in response to supramaximal insulin infusion under these experimental conditions. On the other hand, the mean glucose infusion rate during the last 60 minutes of the insulin clamp in the hepatectomy group was significantly decreased compared with that in the sham-operated group, 57.11 versus 46.29 mumol.kg-1.min-1, respectively (P < .05). Consequently, the maximal capacity of the liver of the anesthetized dog to clear glucose in response to supramaximal insulin infusion appears to be approximately 10.8 mumol.kg-1.min-1, which is about 20% of the total glucose infused. Isotopic data showed that most hepatic glucose uptake was oxidized. In contrast, most peripheral glucose uptake appeared to be stored as glycogen.

Effect of aging on glycogen synthesis in liver of starved-refed rats

Glycogen content in the livers of young, adult and old rats starved for 40 h and refed with a standard diet for 1, 2 or 4 h was determined. In the fed animals the amount of liver glycogen was similar in all the age groups. Fasting depleted liver glycogen almost completely. Chow refeeding led to glycogen synthesis in the livers of young and adult rats; however, glycogen deposition in the livers of old animals was delayed by 1 h. The amount of liver glycogen replenished in the rats refed for 4 h was similar in all the age groups. Plasma glucose concentrations were similar in all age groups of the fed, starved and refed rats. The mechanisms of age-dependent delay of glycogen synthesis in the liver of starved-refed rats are discussed.

Mechanisms underlying enhanced glycogenolysis in livers of 3,5,3'-triiodothyronine-treated rats

Rats trained on a diurnal controlled meal-feeding schedule and injected with a single dose of 3,5,3'-triiodothyronine (T3) failed to accumulate liver glycogen and incorporated less D-[6-3H]glucose into glycogen than normally observed during the feeding period. In the experimental group, the concentration of liver adenosine 3',5'-cyclic monophosphate (cAMP) did not fall during feeding and the pattern of activities of glycogen phosphorylase, glycogen synthase, and phosphorylase kinase remained conductive to glycogenolysis. Liver lysosomal alpha-glucosidase activity normally fell during feeding periods. After T3 treatment the activities of alpha-glucosidase and two lysosomal cathepsins (B1 and D) were elevated. The evidence suggests that T3 may induce both liver phosphorylase kinase and lysosomal alpha-glucosidase. This outcome of T3 excess, in concert with previously described T3-inducible systems, provides a plausible explanation for the failure of glycogen accumulation in this experimental model.