The effect of inositol trisphosphate on Ca2+ fluxes in insulin-secreting tumor cells.

An early event associated with the stimulation of various secretory cells is the breakdown of phosphatidylinositol 4,5-bisphosphate and the mobilization of cellular calcium. Hydrolysis of this inositol lipid by a phosphodiesterase produces inositol trisphosphate (InsP3), a small water-soluble molecule which may serve a messenger function to release Ca2+ from internal stores. In order to assess the role of inositol lipid breakdown in the stimulation of insulin secretion we have examined the effect of InsP3 on Ca2+ fluxes in three different insulin-secreting tumor cells permeabilized by the addition of saponin. A rapid, transient release of Ca2+ from a non-mitochondrial pool occurred upon addition of InsP3 to all three cell types. Half-maximal Ca2+ release from the RIN-1046-38 and RIN-m5F cells was obtained in the concentration range 0.1-0.2 microM. However, the cells obtained from a transplantable tumor of the Syrian hamster were far more sensitive to InsP3 with half-maximal release being observed at 0.025 microM. A partially purified preparation of vesicles was isolated from this tumor which retained its responsiveness to InsP3. Half-maximal Ca2+ release from the vesicles was obtained at 0.2 microM InsP3. Our data are consistent with a role for InsP3 in mediating the increase in cytosolic free Ca2+ which occurs in response to a number of stimuli that promote the secretion of insulin.