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

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 hours) and hyperglycemic (glucose-infused for 24 hours) rats. The profiles of insulin secretion obtained during one hour of perifusion were markedly different in the two 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 conversion, islets were pulse-labeled with L-[4,5-3H]-leucine for 15 minutes and "chase"-incubated for 30 and 60 minutes. Labeled rat proinsulin and rat insulins in the medium and in the islet extracts were separated by a validated SDS-urea electrophoretic acrylamide procedure following immunoprecipitation. The conversion rate was estimated from the radioactivity in the insulin band, expressed as a per cent 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.