Differential rates of release of newly synthesized and of stored insulin from pancreatic islets.

Abstract

Rat pancreatic islets were maintained in tissue culture (8.3 mM glucose) for 3 days in the presence of [3H]leucine in order to label insulin stores. Alternatively, islets were labeled for 1 h at the end of a 3-day maintenance period in order to label only newly synthesized insulin. During a 24-h chase incubation, 13% of the prelabeled insulin stores (3-day label) had been released. By contrast, 30% of newly synthesized (1-h label) insulin was released during a similar chase period. Since islets were handled identically aside from the respective time of exposure to [3H]leucine, the total immunoreactive islet insulin content and that released into the medium was similar for both groups of islets. The specific radioactivity of the labeled insulin in the medium at the end of the chase period was thus lower than that remaining in the islets for islets prelabeled for 3 days but was higher after the labeling of newly synthesized insulin for 1 h. When the rate of newly synthesized insulin was followed during a 4-...