The inositol trisphosphate receptor (IP3R) is an intracellular calcium channel that mediates the cellular actions of a wide variety of hormones, growth factors, and cytokines. In osteoblastic cell cultures, many bone resorbing hormones increase phosphoinositide turnover, inositol trisphosphate production, mobilization of intracellular calcium, and the secretion of osteoclast recruitment and activating factors. In this study, the effects of 171lestradiol, 1,25-dihydroxyvitamin D 3 (1,25(OHhD3)' phorbol ester, and serum on IP3R mRNA levels were evaluated in osteogenic-osteosarcoma cells and in primary osteoblastic cultures derived from neonatal rat calvaria. Type-specific reverse transcription polymerase chain reaction (RT-PCR) indicated that all cell types evaluated (G-292, U-2 OS, Saos-z, MC3T3-E1, UMR-106, and calvarial osteoblastic cells) express IP3R mRNA type I; G-292, U-2 OS, MC3T3-E1, and calvarial osteoblastic cells also express type n IP3R mRNA; and UMR-106 and the calvarial osteoblastic cells express type ill IP3R mRNA. Northern blot and RT-PCR analyses of human G-292 osteosarcoma cells and rat calvarial osteoblastic cells showed that phorbol ester and serum increase IP3R mRNA levels, whereas 17p-estradiol and 1,25(OHhD3 decrease these levels. In G-292 cells, the effect of 17p-estradiol was not due to accelerated IP3R mRNA degradation and required continued protein synthesis. The results show that multiple IP3R types are expressed in osteoblasts and osteoblastic osteosarcoma cells and that this expression is regulated by 17p-estradiol and other osteoporotic and antiosteoporotic hormones. These findings indicate that hormonal control of IP3R expression may be relevant in the chronic regulation of osteoblast secretory activity. (J