Roles of insulin receptor substrate-1, phosphatidylinositol 3-kinase, and release of intracellular Ca2+ stores in insulin-stimulated insulin secretion in β-cells

The signaling pathway by which insulin stimulates insulin secretion and increases in intracellular free Ca2+ concentration ([Ca2+](i)) in isolated mouse pancreatic beta-cells and clonal beta-cells was investigated. Application of insulin to single beta-cells resulted in increases in [Ca2+](i) that were of lower magnitude, slower onset, and longer lifetime than that observed with stimulation with tolbutamide, Furthermore, the increases in [Ca2+](i) originated from interior regions of the cell rather than from the plasma membrane as with depolarizing stimuli. The insulin-induced [Ca2+](i) changes and insulin secretion at single beta-cells were abolished by treatment with 100 nM wortmannin or 1 mu M thapsigargin; however, they were unaffected by 10 mu M U73122, 20 mu M nifedipine, or removal of Ca2+ from the medium. Insulin-stimulated insulin secretion was also abolished by treatment with 2 mu M bisin-dolylmaleimide I, but [Ca2+](i) changes were unaffected. In an insulin receptor substrate-1 gene disrupted beta-cell tumor line, insulin did not evoke either [Ca2+](i) changes or insulin secretion. The data suggest that autocrine-activated increases in [Ca2+](i) are due to release of intracellular Ca2+ stores, especially the endoplasmic reticulum, mediated by insulin receptor substrate-1 and phosphatidylinositol 3-kinase, Autocrine activation of insulin secretion is mediated by the increase in [Ca2+](i) and activation of protein kinase C.