Pancreatic D-Cell Recognition of D-Glucose: Studies with D-Glucose, D-Glyceraldehyde, Dihydroxyacetone, D-Mannoheptulose, D-Fructose, D-Galactose, and D-Ribose

To investigate how the D-cell recognizes the glucose stimulus, the hormone response to (1) glucose, (2) the trioses glyceraldehyde and dihydroxyacetone, (3) the metabolic blocker, mannoheptulose, and (4) the lowor nonmetabolized sugars galactose, fructose, or ribose were studied using the isolated dog pancreas. We found (1) a sigmoidal relationship between extracellular glucose concentrations and the somatostatin release. The threshold concentration was around 5 mM and the largest increase in somatostatin release occurs between 5 and-10 mM of glucose. (2) Glyceraldehyde at concentrations ranging between 1.25 and 5 mM stimulated the release of somatostatin, whereas the higher concentrations of 10 and 20 mM were suppressive. Dihydroxyacetone (11 mM), also initiated somatostatin release in the absence of glucose. Both of the trioses stimulated B- and inhibited A-cell secretion. (3) Mannoheptulose (5 mM) attenuated somatostatin and insulin secretion to 8.3 mM glucose, while it augmented glucagon output. In contrast, mannoheptulose (5 mM) did not affect D-, A-, or B-cell responses to glyceraldehyde (5 mM) in the absence of glucose. (4) The somatostatin, insulin, and glucagon release remained unchanged when 8.3 mM of either galactose, fructose, or ribose was added. The results suggest that the initiation of glucose-mediated D- as well as Aand B-cell responses depends on the metabolism of the sugar.