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 glycoly-sis 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 analysed using fast protein liquid chromatography. The islets were exposed to 3.3, 5.5 and 12 mol 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 mol l⁻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) sigdicantly increased both intracellular content of inositol 1,4,5-trisphosphate, as well as its metabolite inositol 1,3,4-tris-phosphate. 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.

Key Words:

• glucose
• insulin
• phosphatidylinositol phosphates
• second messenger systems
• signal transduction