Stimulus-secretion Coupling of Glucose-induced Insulin Release. Metabolism of Alpha- and Beta-D-glucose in Isolated Islets

Alpha-D-Glucose is known to exert more marked insulinotropic action than B-D-glucose. Both anomers are phosphorylated at the same rate by rat islet homogenates. The islet glucose-6-phosphate dehydrogenase displays a preferential affinity towards beta-D-glucose-6-phosphate, and this coincides with a higher sorbitol content in the islets exposed to beta-D-glucose. On the contrary, the islet phosphoglucose isomerase is stereospecific for alpha-D-glucose 6-phosphate and, hence, the concentration of glucose 6-phosphate is lower and that of the alpha-anomer to lactate and CO2 is also higher than that of beta-D-glucose. This increased glycolytic flux is associated with a more marked inhibitory action on 14Ca efflux, a more pronounced stimulation of 45Ca net uptake and a higher rate of insulin release in the islets exposed to alpha-D-glucose. The more marked insulinotropic action of alpha- as a distinct from beta-D-glucose is thus compatible with the view that glycolysis represents the key component of the sensor device through which glucose is identified in the pancreatic B-cell as a stimulus for insulin release.