Insulin release: the fuel hypothesis.

The immediate and direct regulation of insulin release by circulating nutrients, especially glucose, is thought to be mediated in the pancreatic B-cell by a sequence of metabolic, ionic, and motile events. On the basis of previous work, it is assumed that the process by which glucose is recognized as an insulinotropic agent entirely depends on the metabolic changes evoked by the sugar in the islet cells. Several factors are considered as possible candidates for the coupling between these metabolic changes and subsequent ionic events such as altered phosphate, chloride, sodium, potassium, and calcium handling. It is acknowledged that changes in the concentrations of glycolytic intermediates and cyclic nucleotides (adenosine- or guanosine-3', 5'-cyclic monophosphate), or both, could play a modulatory role upon stimulated insulin release. However, the initiation of insulin release seems to depend on the generation of two essential coupling factors: H+ and reduced pyridine nucleotides. The changes in H+ fluxes may account for the glucose-induced decrease in K+ and Ca2+ fractional outflow rate, all three parameters displaying hyperbolic-like dose-response curves with half-maximal values at noninsulinotropic glucose concentrations. The changes in NAD(P)H concentration may account for a glucose-induced Ca2+--Ca2+ exchange process due to a change in affinity of a native ionophoretic system. The dose-response curves for these parameters yield a sigmoidal pattern analogous to that which depicts the rate of insulin release at increasing glucose concentrations. It is proposed that such a coupling between metabolic and cationic events is operative in response to other insulinotropic nutrients and that its time course may be relevant to the phasic aspect of insulin release. Thus, the nutrient-induced release of insulin (and possibly other pancreatic hormones), which is essential for the regulation of fuel homeostasis, would depend on the capacity of circulating nutrients to act as a fuel in the islet cells. This concept raises a question as to the existence and nature of feedback mechanisms regulating the metabolic fluxes in the islet cells as a function of their energy expenditure.