The effect of fructose metabolism on the accumulation of inositol phosphates in rat pancreatic islets.

Abstract

The mechanism by which glucose recognition of B cells results in the release of inositol 1,4,5-trisphosphate is not known at present. In pancreatic islets, fructose shares a common metabolic pathway with glucose from the second step of glycolysis and can augment insulin secretion at stimulatory glucose levels. To evaluate the impact of glycolysis on the release of inositol 1,4,5-trisphosphate, we studied the effect of glucose and fructose metabolism on insulin secretion and the activation of inositol-specific phospholipase C, using collagenase digested rat pancreatic islets incorporated with 3H-labelled myo-inositol. Inositol phosphates, generated by the cleavage of phosphatidyl inositol by inositol phospholipase C, were analyzed using fast protein liquid chromatography. The islets were exposed to 3.3, 5.5 and 12 mmol 1(-1) glucose for 45 min in the absence or presence of 10, 20 or 30 mmol 1(-1) fructose, and the amount of insulin released into the medium was measured. Intracellular inositol phosphate accumulation was measured under the same glucose concentrations with 0, 10 and 30 mmol 1(-1) fructose. As expected, fructose alone had no insulinotropic effect, but potentiated the glucose-induced (5.5 and 12 mmol 1(-1)) insulin secretion at concentrations of 10-30 mmol 1(-1). Glucose (12 vs. 3.3 mmol 1(-1)) significantly increased both intracellular content of inositol 1,4,5-trisphosphate, as well as its metabolite inositol 1,3,4-trisphosphate. Fructose, however, had no potentiating effects on the accumulation of inositol phosphates. It is therefore supposed that glucose does not activate inositol-specific phospholipase C via the glycolysis. Further, since fructose did not activate inositol-specific phospholipase C, this stimulation is likely to be induced by glucose as such.