P2Y receptor activation enhances insulin release from pancreatic beta-cells by triggering the cyclic AMP/protein kinase A pathway

Adenine nucleotides stimulate insulin secretion by binding to P2 receptors of the pancreatic beta-cells; the stimulus-secretion coupling is not yet clearly established and may depend on the receptor subtype. The aim of the present study was to further investigate the mechanism whereby P2Y receptor agonists enhance glucose-induced insulin secretion. Experiments were performed in rat pancreatic islets and in the INS-1 secreting cell line in the presence of a slightly stimulating glucose concentration (8.3 mmol/l). In isolated islets, the P2Y receptor agonist ADPbetaS (50 micromol/l) induced a significant fivefold increase in the cyclic AMP (cAMP) content, from 43.4+/-3.7 fmol/10 islets in controls to 210.6+/-12.0; it still induced a 4.5-fold increase in cAMP content in the absence of calcium. In another series of experiments, ADPbetaS (50 micromol/l) significantly increased glucose-induced insulin secretion from 7.7+/-0.6 ng/3 islets in controls to 11.2+/-1.0. The adenylyl cyclase inhibitor SQ 22,536 (9-tetrahydro-2-furanyl]-9 H-purin-6-amine; 100 micromol/l), which was ineffective alone, completely prevented the stimulating effect of ADPbetaS. In a set of experiments in which ADPbetaS increased glucose-induced insulin secretion from 10.0+/-0.7 ng/3 islets to 12.6+/-0.8, the inhibitor of cAMP-dependent protein kinase, TPCK (tos-phe-chloromethylketone; 3 micromol/l), which was ineffective alone, also prevented the stimulating effect of ADPbetaS. In incubated INS-1 cells, the P2Y receptor ligand ATPalphaS increased significantly both the content of cAMP and the release of insulin, in a concentration-dependent manner in the range of 50-150 micromol/l; the insulin release was significantly correlated with the cAMP content. In conclusion, the present results show that P2Y receptor agonists, ADPbetaS and ATPalphaS, amplify glucose-induced insulin secretion by activating beta-cell adenylyl cyclase and the subsequent cAMP/protein kinase A signaling pathway.