Abstract

The desensitization mechanisms that regulate the response to thromboxane A2 (TXA2) were investigated in 1321N1 human astrocytoma cells. Exposure of the cells to 9,11-epithio-11, 12-methanothromboxane A2 (STA2), a stable TXA2 receptor agonist, at a concentration of 1 microM for 30 to 45 min resulted in about a 50% decrease in subsequent STA2-stimulated phosphoinositide hydrolysis and Ca2+ mobilization. However, exposure to STA2 for 0 to 5 hr did not change the binding of [3H]SQ29548, a TXA2 receptor antagonist. Because STA2-induced GTPase activation decreased and the GTP sensitivity in inhibition of [3H]SQ29548 binding by STA2 disappeared after the cells had been exposed to STA2 for 30 min, the TXA2 receptor desensitization during the short-term might result from G-protein-receptor uncoupling. STA2-induced desensitization was specific for the TXA2 receptor and homologous, because SQ29548 suppressed the desensitization and STA2 pretreatment did not affect the response to carbachol, a muscarinic cholinergic receptor agonist. Exposure to STA2 for 24 hr decreased [3H]SQ29548 binding sites to 20 to 30% of control and abolished STA2-stimulated phosphoinositide hydrolysis, indicating that long-term desensitization might induce down-regulation of the TXA2 receptor. However, exposure to STA2 for 1 to 24 hr did not change the level of TXA2 receptor mRNA. These results show that homologous desensitization of the TXA2 receptor in human astrocytoma cells can be divided into two stages; the early stage involves uncoupling of receptors from G-proteins and the late stage involves a loss of receptors in cells. The mRNA levels may not be controlled by stimulation of the TXA2 receptor.