The Stimulus-secretion Coupling of Glucose-induced Insulin Release. XXII. Qualitative and Quantitative Aspects of Glycolysis in Isolated Islets

When isolated islets of Langerhans are suddenly exposed to glucose, the entry of the hexose into islet cells first occurs at a high rate resulting in rapid equilibration of free glucose across the cell membrane; thereafter, the rate of net glucose uptake depends on its metabolism. More than 95% of the glucose taken up by the islets is converted to triosephosphate. The fractional contribution of the sorbitol and pentose-phosphate pathways to such a process does not exceed 10%. The output of lactate from the islets accounts for approximately half of the glycolytic flux. At increasing glucose concentrations up to 4.3 mM, the rate of glycolysis increases towards a first asymptotic value; at higher glucose levels (up to 27.8 mM), a sigmoidal pattern is seen tending towards a second saturation value. The total ATP content of the islets does not correlate with their insulin-secretory activity. It is suggested that, in the process of glucose-induced insulin release, glycolysis may regulate physiological processes possibly located in the micro-environment of the cell boundary.