INSULIN RELEASE DURING FASTING - STUDIES ON ADENYLATE-CYCLASE, PHOSPHODIESTERASE, PROTEIN-KINASE, AND PHOSPHOPROTEIN PHOSPHATASE IN ISOLATED ISLETS OF LANGERHANS OF THE RAT

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-1min-1from 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 affected by fasting. In islets from fasted rats, basal adenylate cyclase activity was not significantly different from that observed in islets from fed controls (control, 15.3 ± 1.2 pmol cAMP mg-1min-1; fasting, 17.6 ± 1.9; P < 0.2). Likewise, the low Kmphosphodiesterase activity was not different in the fasted state (control, 5.7 ± 0.6 pmol cAMP mg-1min-1; fasting, 5.3 ± 0.6; P < 0.3). High Kmphosphodiesterase activity was decreased by 13% in islets from fasted animals (control, 425 ± 29 pmol cAMP mg-1min-1; fasting, 369 ± 26; P < 0.02). Basal protein kinase activity in islets from fasted rats was similar to that of controls (control, 182 ± 23 pmol32PO4mg-1min-1; fasted, 174 ± 24; P < 0.3). In the presence of 1 pM cAMP, activity in both groups increased by 180% (control, 502 ± 51 pmol32PO4mg-1min-1; fasted, 484 ± 61; P < 0.4). Phosphoprotein phosphatase activity was decreased by 12.5% in the fasted state (control, 8.8 ± 0.5 pmol ;32PO4mg-1min-1; fasted, 7.7 ± 0.5; P < 0.01). From the results of these enzyme assays it appears that changed enzyme activity in the adenylate cyclase-cAMP-protein phosphorylation system plays little role in the decreased insulin release observed in the fasted state. The difference in the insulin secretory response between D-glucose and D-glyceraldehyde in islets from fasted animals supports the idea that the major ratelimiting step in stimulus-secretion coupling in the fasted state is before the metabolism of the trioses. © 1979 by The Endocrine Society.