Effect of receptor number on desensitization of the mouse thromboxane receptor.

Desensitization of G-protein coupled receptors limits the physiologic effects of an agonist. Short-term desensitization mechanisms are critically dependent on receptor phosphorylation by protein kinases. The effectiveness of these regulatory mechanisms might be limited by substrate (receptor) availability. To investigate the role of receptor number in the desensitization of G-protein coupled receptors, we transfected a mouse mesangial cell line with a genomic clone encoding the mouse thromboxane A2 (TxA2) receptor and obtained cell lines that expressed low (approximately 250-500 fmol/mg protein) or high (2500-4000 fmol/mg protein) levels of TxA2 receptors. Activation of TxA2 receptors stimulated phosphoinositide (PI) hydrolysis and increased intracellular calcium ([Ca2+]i) levels. Prior exposure to the TxA2 agonist (15S)-hydroxy-11alpha,9alpha-(epoxymethano)prosta-5Z,+ ++13E-dienoic acid (U46619) reduced subsequent (15S)-hydroxy- 11alpha,9alpha-(epoxymethano)prosta-5Z,13E-dieno ic acid-induced increases in inositol trisphosphates and intracellular calcium levels by approximately 50% in clones expressing low numbers of TxA2 receptors, but had little effect on TxA2 receptor responsiveness in clones expressing high receptor numbers. Failure of TxA2 receptors to desensitize caused sustained increases in intracellular calcium levels and phosphoinositide hydrolysis. Thus, homologous desensitization of TxA2 receptors is attenuated in cells expressing high levels of receptors for TxA2. These data suggest that receptor number plays a key role in the short-term regulation of G-protein coupled receptors.