Immediate and delayed effects of d-fructose upon insulin, somatostatin, and glucagon release by the perfused rat pancreas

Novel information was recently provided concerning the reciprocal effects of d-glucose and d-fructose upon their respective metabolism in rat pancreatic islets. In the light of such findings, this study aims at comparing the effects of d-glucose and d-fructose on insulin, somatostatin, and glucagon release from the isolated perfused rat pancreas. A rise in d-glucose concentration from 3.3 to 5.0 or 7.3 mM or the administration of d-fructose (17 and 40 mM) in the presence of 3.3 mMd-glucose stimulated insulin release in a concentration-related manner, but failed to affect somatostatin output. The secretion of glucagon was decreased in all cases. The secretory response to l-arginine (5 mM), 25 min after restoring the basal concentration of d-glucose, was more markedly affected, in terms of potentiation of insulin and somatostatin release and reduction of glucagon output, after prior administration of d-fructose than after a prior increase in d-glucose concentration. These findings argue against any major role for a paracrine regulation of hormonal release and, instead, are consistent with a causal link between metabolic and secretory events in the islet cells. Nevertheless, the present results emphasize differences in the response of distinct pancreatic endocrine cell types to the same or distinct hexoses.