title: Differential desensitization of thromboxane A2 receptor subtypes.
abstract: Two subtypes of the thromboxane A2 (TxA2) receptor (TxA2R-E and TxA2R-P), which differ in their alternatively spliced cytoplasmic tails, have been identified. The initial concentration of the TxA2 mimetic IBOP required to reduce peak intracellular Ca2+ concentration ([Ca2+]i) induced by a second addition of IBOP (100 nmol/L) was similar (IC50 for TxA2R-E and TxA2R-P, 0.46 +/- 0.16 and 0.40 +/- 0.07 nmol/L) in fibroblasts overexpressing either the TxA2R-E or -P subtype. Although the number of TxA2 binding sites decreased in TxA2R-P cells after prolonged stimulation with a TxA2 mimetic, those in the TxA2R-E cells increased markedly. To determine whether the mechanism for desensitization differs between subtypes, the effect of activation of protein kinase C (PKC) or cAMP-dependent kinase on TxA2-induced [Ca2+]i mobilization was measured. Forskolin reduced the IBOP-induced peak [Ca2+]i in neither TxA2R-E nor TxA2R-P cells; however, treatment with phorbol esters (IC50, 0.57 +/- 0.70 nmol/L) strongly prevented IBOP-mediated [Ca2+]i rise in TxA2R-E but not in TxA2R-P cells. Desensitization of TxA2R-E by phorbol esters was prevented by the PKC inhibitor calphostin C or by downregulation of PKC-alpha. Thus, the response of TxA2R-E to prolonged stimulation differs from that of TxA2R-P in both the regulation of the number of binding sites and the mechanism for desensitization; agonists that activate PKC-alpha might interfere with TxA2R-E-mediated signaling.
authors.affiliation: Vascular Biology Unit, Beth Israel Hospital, Boston, Mass, USA.
authors.forename: M
authors.initials: M
authors.lastname: Yukawa
authors.forename: R
authors.initials: R
authors.lastname: Yokota
authors.forename: R T
authors.initials: RT
authors.lastname: Eberhardt
authors.forename: L
authors.initials: L
authors.lastname: von Andrian
authors.forename: J A
authors.initials: JA
authors.lastname: Ware
mesh_terms: D019796:15-Hydroxy-11 alpha,9 alpha-(epoxymethano)prosta-5,13-dienoic Acid; D000818:Animals; D016466:CHO Cells; D002118:Calcium / Q000378:metabolism; D005576:Colforsin / Q000494:pharmacology; D006224:Cricetinae; D017868:Cyclic AMP-Dependent Protein Kinases / Q000378:metabolism; D004789:Enzyme Activation; D004791:Enzyme Inhibitors / Q000494:pharmacology; D009281:Naphthalenes / Q000494:pharmacology; D011450:Prostaglandin Endoperoxides, Synthetic / Q000494:pharmacology; D011493:Protein Kinase C / Q000037:antagonists & inhibitors / Q000378:metabolism; D017482:Receptors, Thromboxane / Q000819:agonists / Q000378:metabolism*; D013755:Tetradecanoylphorbol Acetate / Q000494:pharmacology; D013928:Thromboxane A2 / Q000031:analogs & derivatives / Q000494:pharmacology
chemical_list: D004791:Enzyme Inhibitors; D009281:Naphthalenes; D011450:Prostaglandin Endoperoxides, Synthetic; D017482:Receptors, Thromboxane; D005576:Colforsin; D013928:Thromboxane A2; D019796:15-Hydroxy-11 alpha,9 alpha-(epoxymethano)prosta-5,13-dienoic Acid; D017868:Cyclic AMP-Dependent Protein Kinases; D011493:Protein Kinase C; C058819:calphostin C; D013755:Tetradecanoylphorbol Acetate; D002118:Calcium