Gastrointestinal Hormones and Function of Pancreatic Islets: Studies on Insulin Secretion,3H-Leucine Incorporation and Intracellular Free Leucine Pool in Isolated Pancreatic Mouse Islets
Secretion and biosynthesis of (pro-) insulin were studied in isolated pancreatic mouse islets in the presence of secretin, pancreozymincholecystokinin, gastrin and glucagon, at 0, 100 and 300 mg⁄100 ml glucose. Incorporation of aHleucine into the proinsulin and insulin fraction obtained after gel nitration of the islet proteins on a Sephadex G 50 fine column was taken as parameter of insulin synthesis. Specific radioactivity of free intracellular leucine pool in the isolated islets was determined under different conditions. In the absence of glucose, pancreozymin was the only substance to stimulate insulin secretion.When glucose was present in the medium, glucagon was able to potentiate insulin output. Secretin and gastrin did not change insulin release. Insulin synthesis, found to be stimulated by glucose like insulin secretion, was unaffected by all 3 gastrointestinal hormones whether or not glucose was present. (Pancreatic) glucagon, on the other hand, potentiated glucose-induced insulin synthesis. Specific radioactivity of free intracellular leucine pool was found to be not changed by different glucose concentrations or by the addition of glucagon (and tolbutamide). Obviously, insulin secretion can be stimulated by a comparatively large number of agents whereas in vitro insulin synthesis appears to be regulated relatively uniformly. Glucose is of utmost importance for the latter process in which glucagon may be involved, too. (Endocrinology94: 248, 1974)