Secretion of insulin in a perifusion system and conversion of proinsulin to insulin by pancreatic islets from hyperglycemic rats

Abstract

The secretory pattern of insulin and the rate of conversion of proinsulin to insulin were studied in isolated pancreatic islets from normoglycemic (buffer-infused for 24 h) and hyperglycemic (glucose-infused for 24 h) rats. The profiles of insulin secretion obtained during 1 h of perifusion were markedly different in the 2 groups. The rate of insulin secretion by islets from the hyperglycemic rats was initially very high but progressively declined during the late period of the perifusion. The reverse pattern was found with the islets from buffer-infused rats. For the estimation of the rate of proinsulin to insulin conversion, islets were pulse-labeled with L-[4,5-3H]-leucine for 15 min and chase-incubated for 30 and 60 min. Labeled rat proinsulins and rat insulins in the medium and in the islet extracts were separated by a validated SDS[sodium dodecyl sulfate]-urea electrophoretic acrylamide procedure following immunoprecipitation. The conversion rate was estimated from the radioactivity in the insulin band, expressed as a percent of the radioactivity in the proinsulin + insulin bands. Islets from hyperglycemic rats converted newly synthesized proinsulin to insulin at significantly higher rates than did control islets.