A comparison of the utilization rates and hormone-releasing actions of glucose, mannose, and fructose in isolated pancreatic islets.

The abilities of glucose, mannose, and fructose to serve as substrates for glycolysis were compared with the potencies of these hexoses to stimulate hormone secretion. Glucose and mannose induced insulin release at threshold levels of 4 and 10 mM and were effective half-maximally at 8 and 15 mM, respectively. Maximal release with glucose and mannose occurred at 15 and 20 mM, respectively. At their threshold levels for insulin release, glucose and mannose were both metabolized at rates of approximately 50 pmol/ islet/h. At levels causing half-maximal release, the glycolytic rates were 80 to 85 pmol/islet/h. At substrate levels causing maximal hormone release, the glycolytic rates reached 100 to 110 pmol/islet/h for both hexoses. Fructose, even at 50 mM, did not stimulate insulin release. Halfmaximal usage (15 pmol/islet/h) occurred with 16 mM fructose, while maximal usage (30 pmol/islet/h) was obtained at 27.5 mM fructose. Fructose and glucose usage were not interfered with by a lo-fold excess of glucose and fructose, respectively. Fructose was, however, capable of augmenting insulin secretion in the presence of substimulatory or stimulatory glucose. This potentiation could be predicted if one assumed that glycolytic rates of the two hexoses were additive. The results strongly support the concept that the glycolytic degradation of hexoses controls hormone secretion.