The inositol trisphosphate receptor (IP3R) is an intracellular calcium channel that mediates the cellular actions of many hormones, growth factors, and cytokines. In osteoblastic cell cultures, many bone resorbing hormones increase inositol trisphosphate production, mobilization of intracellular calcium, and secretion of osteoclast recruitment and activating factors. The effects of 17B-estradiol, 1,25-dihydroxyvitamin D3, phorbol esters and serum on IP3R mRNA levels were evaluated in osteogenic-osteosarcoma cells (G-292, U-2 OS, Saos-2, MC3T3-El and UMR-106) and in primary osteoblastic cultures derived from neonatal rat calvaria. Type specific RT-PCR indicated that all cell types evaluated express IP3R mRNA type I; G-292, U-2 OS, MC3T3-El, and calvarial osteoblastic cells express type II IP3R mRNA; and UMR-106 and the calvarial osteoblastic cells express type III IP3R gene. Northern blot analysis indicated that phorbol ester and serum increase steady state IP3R mRNA expression in G-292 cells and in calvarial osteoblastic cells. Further characterization of IP3R gene expression in G-292 cells indicated that 17B-estradiol and 1,25-dihydroxyvitamin D3 decrease IP3R mRNA levels. The effect of 17B-estradiol was not due to accelerated IP3R mRNA degradation and required continued protein synthesis. The results indicate that IP3R expression in osteoblastic osteosarcoma cells is affected by 17B-estradiol and other osteoporotic and anti-osteoporotic hormones and may be important in the chronic regulation of osteoblast secretory activity.