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Modulation of glucagon-induced glucose production by dexfenfluramine in rat hepatocytes
1Department of Nutrition, University of Montreal, Québec, Canada.
Abstract:
The mechanism of the antihyperglycaemic action of dexfenfluramine (DEXF) was investigated in isolated rat hepatocytes exposed to glucagon. Preincubation of hepatocytes with DEXF caused a dose-dependent inhibition of cyclic AMP formation by 100 nM glucagon (Ki = 0.29 mM) that was almost complete at 1 mM DEXF. Surprisingly, glucagon-induced phosphorylase activation was not affected by DEXF despite the significant drop in cyclic AMP levels. Glucose production stimulated by glucagon was inhibited by up to 48% by 1 mM DEXF, and the rate of glucose production correlated positively with the steady-state concentration of glucose 6-phosphate. DEXF also partially restored lactate + pyruvate production which was abolished by an optimal concentration of glucagon. Although DEXF was not able to prevent the inactivation of pyruvate kinase by glucagon, the lack of further accumulation of phosphoenolpyruvate in DEXF-treated cells supports the conclusion that the flux through pyruvate kinase is stimulated, probably via the increase in fructose 2,6-bisphosphate, thereby increasing glycolysis. Our results thus indicate that DEXF counteracts the inhibition of glycolysis by glucagon and that this property might contribute to the antihyperglycaemic effect of this drug. Furthermore, this study shows that, in the presence of the drug, glucagon caused phosphorylase activation and pyruvate kinase inactivation without a significant increase in cyclic AMP levels.
Insights
Dexfenfluramine (DEXF) inhibits glucagon-induced cyclic AMP formation in rat hepatocytes, counteracting glycolysis inhibition. This action may contribute to DEXF's antihyperglycemic effects.
Area of Science:
- Biochemistry
- Pharmacology
- Hepatology
Background:
- Glucagon is a key hormone regulating blood glucose levels.
- Dexfenfluramine (DEXF) is known for its antihyperglycemic properties.
- The precise mechanism of DEXF's action on glucose metabolism is not fully understood.
Purpose of the Study:
- To investigate the mechanism of dexfenfluramine's (DEXF) antihyperglycemic action.
- To elucidate DEXF's effects on isolated rat hepatocytes stimulated by glucagon.
- To determine how DEXF influences key enzymes and metabolites in glucose production and glycolysis.
Main Methods:
- Isolated rat hepatocytes were preincubated with varying concentrations of DEXF.
- Hepatocytes were exposed to glucagon to stimulate cellular responses.
- Measurements included cyclic AMP levels, phosphorylase activation, glucose production, lactate + pyruvate production, and enzyme activity (pyruvate kinase).
Main Results:
- DEXF dose-dependently inhibited glucagon-induced cyclic AMP formation.
- Glucagon-induced phosphorylase activation was unaffected by DEXF, despite reduced cyclic AMP.
- DEXF inhibited glucagon-stimulated glucose production and partially restored lactate + pyruvate production, indicating increased glycolysis.
- DEXF stimulated pyruvate kinase flux, likely via fructose 2,6-bisphosphate, counteracting glucagon's inhibitory effects.
Conclusions:
- Dexfenfluramine (DEXF) counteracts glucagon's inhibition of glycolysis in rat hepatocytes.
- DEXF's ability to enhance glycolysis may contribute to its antihyperglycemic effect.
- Glucagon can induce phosphorylase activation and pyruvate kinase inactivation independently of significant cyclic AMP increases in the presence of DEXF.