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 β-cells and clonal β-cells was investigated. Application of insulin to single β-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 β-cells were abolished by treatment with 100 nM wortmannin or 1 μM thapsigargin; however, they were unaffected by 10 μM U73122, 20 μM nifedipine, or removal of Ca²⁺ from the medium. Insulin-stimulated insulin secretion was also abolished by treatment with 2 μM bisindolylmaleimide I, but [Ca²⁺](i) changes were unaffected. In an insulin receptor substrate-1 gene disrupted β-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.