Expression of inositol 1,4,5-trisphosphate receptors changes the Ca2+ signal of Xenopus oocytes

The receptors for the second messenger inositol 1,4,5-trisphosphate [Ins(1,4,5)P-3] form a family of closely related proteins that play an important role in regulating the free intracellular Ca2+ concentration. To test the hypothesis that changing the expression level of Ins(1,4,5)P-3 receptors could alter the Ins(1,4,5)P-3-mediated Ca2+ signal, we overexpressed Ins(1,4,5)P-3 receptor type 1 (InsP(3)R-1) or type 3 (InsP(3)R-3) in Xenopus laevis oocytes. Expression of InsP(3)R-1 increased the velocity of the propagating waves of intracellular Ca2+ release but did not affect the Ins(1,4,5)P-3-induced entry of extracellular Ca2+ across the plasma membrane. In contrast, expression of InsP(3)R-3 did not affect Ins( 1,4,5)P-3-induced release of intracellular Ca2+ but markedly increased the magnitude and duration of Ca2+ influx. Immunolocalization studies revealed InsP(3)R-3 at the endoplasmic reticulum, with a relatively stronger signal at or near the plasma membrane. The results suggest that changing the expression level of an InsP(3)R can alter the Ins(1,4,5)P-3-mediated Ca2+ signal and that InsP(3)R-1 and InsP(3)R-3 may have different biological functions.