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

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Abstract
The adenylate cyclase-c[cyclic]AMP system and related phosphorylating and dephosphorylating enzymes have been studied in islets of Langerhans from pregnant and nonpregnant Wistar rats to ascertain if changes in these enzymes might be related to the increase in insulin secretion observed late in pregnancy. Islets of Langerhans were isolated by collagenase digestion from 20 day pregnant rats and age-matched female controls. Islets of each group were perifused to measure rates and patterns of insulin release when they were exposed to 2.8 mM, and subsequently 16.7 mM, D-glucose. The remaining islets were sonicated and assayed for adenylate cyclase, phosphodiesterase, phosphoprotein phosphatase, and protein kinase activities. Basal insulin secretion under perfusion conditions (2.8 mM glucose) was the same in both groups of islets (9.2 .+-. 2.4 pg insulin released .cntdot. islet-1 .cntdot. min-1 from pregnant rats compared with 10.2 .+-. 1.8 for controls). In response to 16.7 mM glucose, both phases of insulin release were increased in islets from pregnant rats relative to controls. Within 3 min of the change from low to high glucose, the increase in insulin release in islets from pregnant rats was double that seen in control islets (P < 0.02). Summing the 1st and 2ndphase responses to high glucose, the islets from pregnant rats had a 75% and 130% greater response, respectively, than did control islets. Basal adenylate cyclase activity was 13.5 .+-. 1.6 pmol cAMP .cntdot. mg-1 .cntdot. min-1 for controls and 19.3 .+-. 2.1 pregnancy (43% increase in activity, P < 0.05). High Km phosphodiesterase activity (control, 364 .+-. 18 pmol cAMP converted .cntdot. mg-1 .cntdot. min-1 compared with 399 .+-. 18) and low Km phosphodiesterase activity (control, 4.7 .+-. 0.4 vs. 4.1 .+-. 0.3 in pregnancy) were unchanged. Basal protein kinase activity was increased 34% in islets from pregnant animals (176 .+-. 12 compared with 131 .+-. 9 pmol 32PO4 incorporated .cntdot. mg-1 .cntdot. min-1, P < 0.01). cAMP (1 .mu.M) increased the protein kinase activity by 200% in islets sonicates from both pregnant and control rats. Thus, there was a 31% increase in cAMP dependent protein kinase activity from pregnant rats (P < 0.02). Phophoprotein phosphatase activity was measured, using [32P] spectrin II phosphate as substrate. No change in activity was observed (controls, 11.9 .+-. 0.7 pmol 32PO4 released .cntdot. mg-1 min-1 vs. 12.3 .+-. 0.7 in pregnancy).