Direct effects of glucose on proinsulin synthesis and processing during desensitization.
Prolonged exposure of isolated islets to glucose (11 mM) results in desensitization of insulin secretion. In this study, both glucose-regulated proinsulin biosynthesis and its processing during glucose-induced desensitization were examined at critical periods during 24 h. Although insulin secretion declined at 24 h to one third the 3 h maximum rate, the total rate of proinsulin biosynthesis, assessed by [3H]leucine incorporation, was unchanged at 0, 3, and 24 h of glucose (11 mM) exposure. Total insulin recovery measured by immunoreactive insulin in islets and media after 24 h was approximately 172% of the initial islet content. After correction for insulin degradation, and since proinsulin biosynthesis was unchanged, the synthesis rate was calculated to be a constant 3.5 +/- 1.1%/h of the initial islet content. Results suggest that desensitization may occur at the release mechanism rather than at a step in glucose metabolism common for the stimulation of synthesis and secretion. In contrast, the conversion rate of proinsulin to insulin progressively increased with prolonged prior glucose exposure, the major increase occurring by 3 h of glucose preincubation. Low glucose (2 mM), when used during the 3-h prelabeling period, did not affect the conversion rate. Furthermore, cycloheximide added during the 3-h preexposure to glucose (11 mM) completely prevented glucose-induced activation of the conversion process. These results indicate that the conversion rate of proinsulin to insulin is a glucose-regulated process requiring synthesis of a pool of either converting enzyme(s) or other regulating protein before initiation of proinsulin synthesis.