Effects of vasopressin on fructose and glycogen metabolism in hepatocytes from fed and fasted rats

Glycogen synthesis from glucose and fructose in hepatocytes from diabetic rats

In rat hepatocytes, the basal glycogen synthase activation state is decreased in the fed and diabetic states, whereas glycogen phosphorylase a activity decreases only in diabetes. Diabetes practically abolishes the time- and dose-dependent activation of glycogen synthase to glucose especially in the fed state. Fructose, however, is still able to activate this enzyme. Glycogen phosphorylase response to both sugars is operative in all cases. Cell incubation with the combination of 20 mM glucose plus 3 mM fructose produces a great activation of glycogen synthase and a potentiated glycogen deposition in both normal and diabetic conditions. Using radiolabeled sugars, we demonstrate that this enhanced glycogen synthesis is achieved from both glucose and fructose even in the diabetic state. Therefore, the presence of fructose plays a permissive role in glycogen synthesis from glucose in diabetic animals. Glucose and fructose increase the intracellular concentration of glucose 6-phosphate and fructose reduces the concentration of ATP. There is a close correlation between the ratio of the intracellular concentrations of glucose 6-phosphate and ATP (G6-P/ATP) and the activation state of glycogen synthase in hepatocytes from both normal and diabetic animals. However, for any given value of the G6-P/ATP ratio, the activation state of glycogen synthase in diabetic animals is always lower than that of normal animals. This suggests that the system that activates glycogen synthase (synthase phosphatase activity) is impaired in the diabetic state. The permissive effect of fructose is probably exerted through its capacity to increase the G6-P/ATP ratio which may partially increase synthase phosphatase activity, rendering glycogen synthase active.

Enhancement of glycogen concentrations in primary cultures of rat hepatocytes exposed to glucose and fructose

Glycogen synthesis in isolated hepatocytes can occur from glucose both by a direct mechanism and by an indirect process in which glucose is first metabolized to C3 intermediates before use for glycogenesis via gluconeogenesis. We studied the incorporation into glycogen of glucose and the gluconeogenic substrate, fructose, in primary cultures of hepatocytes from fasted rats. In the presence of insulin, both glucose and fructose promoted net deposition of glycogen; however, fructose carbon was incorporated into glycogen to a greater extent than that from glucose. When glucose and fructose were administered simultaneously, the glycogenic utilization of glucose was stimulated 2-3-fold, and that of fructose was increased by about 50%. At constant hexose concentrations, the total incorporation of carbon, and the total accumulation of glycogen mass, from glucose and fructose when present together exceeded that from either substrate alone. Fructose did not change the relative proportion of glucose carbon incorporated into glycogen via the indirect (gluconeogenic) mechanism. The synergism of glucose and fructose in glycogen synthesis in isolated rat hepatocytes in primary culture appears to result from a decrease in the rate of degradation of newly deposited glycogen, owing to (i) decreased amount of phosphorylase a mediated by glucose and (ii) noncovalent inhibition of residual phosphorylase activity by some intermediate arising from the metabolism of fructose, presumably fructose 1-phosphate.

Quantitative analysis of intermediary metabolism in hepatocytes incubated in the presence and absence of glucagon with a substrate mixture containing glucose, ribose, fructose, alanine and acetate

Hepatocytes were isolated from the livers of fed rats and incubated, in the presence and absence of 100 nM-glucagon, with a substrate mixture containing glucose (10 mM), fructose (4 mM), alanine (3.5 mM), acetate (1.25 mM), and ribose (1 mM). In any given incubation one substrate was labelled with 14C. Incorporation of 14C into glucose, glycogen, CO2, lactate, alanine, glutamate, lipid glycerol and fatty acids was measured after 20 and 40 min of incubation under quasi-steady-state conditions [Borowitz, Stein & Blum (1977) J. Biol. Chem. 252, 1589-1605]. These data and the measured O2 consumption were analysed with the aid of a structural metabolic model incorporating all reactions of the glycolytic, gluconeogenic, and pentose phosphate pathways, and associated mitochondrial and cytosolic reactions. A considerable excess of experimental measurements over independent flux parameters and a number of independent measurements of changes in metabolite concentrations allowed for a stringent test of the model. A satisfactory fit to the data was obtained for each condition. Significant findings included: control cells were glycogenic and glucagon-treated cells glycogenolytic during the second interval; an ordered (last in, first out) model of glycogen degradation [Devos & Hers (1979) Eur. J. Biochem. 99, 161-167] was required in order to fit the experimental data; the pentose shunt contributed approx. 15% of the carbon for gluconeogenesis in both control and glucagon-treated cells; net flux through the lower Embden-Meyerhof pathway was in the glycolytic direction except during the 20-40 min interval in glucagon-treated cells; the increased gluconeogenesis in response to glucagon was correlated with a decreased pyruvate kinase flux and lactate output; fluxes through pyruvate kinase, pyruvate carboxylase, and phosphoenolpyruvate carboxykinase were not coordinately controlled; Krebs cycle activity did not change with glucagon treatment; flux through the malic enzyme was towards pyruvate formation except for control cells during interval II; and 'futile' cycling at each of the five substrate cycles examined (including a previously undescribed cycle at acetate/acetyl-CoA) consumed about 26% of cellular ATP production in control hepatocytes and 21% in glucagon-treated cells.