Sequential analysis of the releasing and fuel function of glucose in isolated perifused pancreatic islets.

Isolated islets were continuously perifused with glucose to test their secretory capacity in a dynamic fashion, and were subsequently transferred to an incubation vial to measure their capacity for metabolizing glucose. Insulin release was measured by radioimmunoassay and metabolism of glucose by determining the rate of 3H2O formation from glucose tritiated on carbon atom 2 or 5 and by lactate accumulation. Insulin release was induced by glucose at a threshold of 5 mM, was half maximal at 8 mM, maximal at 15 mM and showed biphasic kinetics, which is consistent with published data. However, in contrast to most previous reports, utilization of glucose and lactate formation showed hyperbolic concentration dependency curves. Maximal and half maximal rates of glucose use were obtained at 30 and 8 mM, respectively, and lactate formation reached highest rates at 8 mM glucose. Physiological changes of glucose levels (from 5 to 10 mM) increased hormone release 4-fold (from 0.49 to 1.94 muU/islet/min) whereas glucose use was changed only slightly (from 52 to 75 pmol/islet/h), and lactate formation not at all. These data show that there is only limited association between metabolic and insulin releasing efficiency of glucose in pancreatic islets in vitro and also implicate and threshold phenomenon triggered by either a glucose metabolite or glucose itself.