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 Ca 2+ 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 Ca 2+ signal, we overexpressed Ins(1,4,5)P3 receptor type 1 (InsP3R-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 Ca 2+ release but did not affect the Ins(1,4,5)P 3 -induced entry of extracellular Ca 2+ 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 Ca 2+ but markedly increased the magnitude and duration of Ca 2+ 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 InsP3R can alter the Ins(1,4,5)P3-mediated Ca 2+ signal and that InsP 3 R-1 and InsP3R-3 may have different biological functions.