Insulin Release during Fasting: Studies on Adenylate Cyclase, Phosphodiesterase, Protein Kinase, and Phosphoprotein Phosphatase in Isolated Islets of Langerhans of the Rat*

Abstract

The role of enzymes of the adenylate cyclasecAMP- protein phosphorylation system in the decreased insulin release observed during fasting has been investigated. Islets of Langerhans were isolated by collagenase digestion from paired 48-h fasted male Wistar rats and fed controls. Islets of both groups were perifused to measure rates and patterns of insulin release when exposed to 16.7 mM D-glucose or 10 mM D-glyceraldehyde. Similarly, islets from both groups were sonicated and assayed for adenylate cyclase, phosphodiesterase, protein kinase, cAMP-stimulated protein kinase, and phosphoprotein phosphatase activities. Basal insulin secretion under perifusion conditions with 2.8 mM D-glucose was not different in the two groups of islets (13.7 ± 1.6 pg insulin released islet-1 min-1 from fasted rats compared with 16.7 ± 1.2 for controls; P < 0.2). In response to 16.7 mM Dglucose, insulin release was diminished in islets from fasted animals by 50%, whereas D-glyceraldehyde-stimulated insulin release was not affe...