Thromboxane A₂ (TXA₂) receptor subtypes and their signal transduction were examined in 1321N1 human astrocytoma cells. Placental and endothelial types of TX₂ receptor mRNA were found in astrocytoma cells by RT-PCR procedure. Using immunoaffinity column conjugated with anti-TXA₂ receptor antibody, two TXA₂ receptors (58 and 55 kDa) were partially purified. The partially purified TXA₂ receptor fraction contained G_q/11 and G₁₂. The incubation of the cells with dibutyryl cyclic AMP (dbcAMP) caused a differentiation of human astrocytoma cells. DbcAMP treatment resulted in the reduction of Ca²⁺ elevation and phosphoinositide hydrolysis induced by TXA₂ receptor agonist. Whereas the responsiveness of Ca²⁺ signaling was weakened by dbcAMP treatment, phosphorylation of mitogen-activated protein kinase was increased in dbcAMP-treated cells. These results suggest that human astrocytoma cells express placental and endothelial TXA₂ receptors. The TXA₂ receptors were coupled with G_q/11 and G₁₂. DbcAMP treatment discriminates TXA₂ receptor-mediated MAPK activation from the Ca²⁺ signaling pathway.