Type I, II, and III inositol 1,4,5-trisphosphate receptors are unequally susceptible to down-regulation and are expressed in markedly different proportions in different cell types.

The type I inositol 1,4,5-trisphosphate (InsP3) receptor can be rapidly depleted from cells during stimulation of phosphoinositide hydrolysis because its degradation is accelerated (Wojcikiewicz, R. J. H., Furuichi, T., Nakade, S., Mikoshiba, K., and Nahorski, S. R. (1994) J. Biol. Chem. 269, 7963-7969). The present study examines the regulatory properties of type II and III InsP3 receptors. Initially, the relative abundance of InsP3 receptors was defined in a range of cell types by quantitative immunoblotting. These studies showed that the proportions in which type I, II, and III InsP3 receptors are expressed differs greatly and that some cells (for example, AR4-2J rat pancreatoma cells) express all three receptors. Analysis of the effects of cholecystokinin and bombesin on AR4-2J cells showed that each of the InsP3 receptors could be down-regulated during activation of phosphoinositide hydrolysis, but that depletion of the type II receptor was limited. Such a discrepancy was also seen in rat cerebellar granule cells and was found to result from the type II receptor being relatively resistant to degradation. In conclusion, type I, II, and III receptors can all be down-regulated, but with different characteristics. As the relative abundance of InsP3 receptors is extremely variable, the extent to which activation of the down-regulatory process alters intracellular signaling will vary depending on which InsP3 receptors are expressed.