title: Agonist-induced phosphorylation of human platelet TXA2/PGH2 receptors.
abstract: Thromboxane A2 (TXA2) and its precursor prostaglandin H2 (PGH2) stimulate platelet activation through interaction with TXA2/PGH2 receptors. We and others have shown that these receptors undergo homologous desensitization upon prolonged exposure to thromboxane A2 mimetics. Phosphorylation of receptors has previously been reported to be an important mechanism for receptor desensitization. In the present study we examined the possibility that homologous desensitization of human platelet TXA2/PGH2 receptors may involve phosphorylation. The ATP pool of human platelets was metabolically prelabeled with 32Pi and the labeled platelets were subsequently exposed for 1 and 10 min to the stable TXA2/PGH2 mimetic, U46619 (1 microM). TXA2/PGH2 receptors were purified approx. 2000-fold by affinity and wheatgerm lectin chromatography and subjected to SDS-PAGE followed by autoradiography. A phosphorylated plasma membrane glycoprotein (M(r) = 50-57 kDa) was detected with characteristics similar to the TXA2/PGH2 receptor. This glycoprotein was found to be phosphorylated in the unstimulated state but phosphorylation was increased by exposure to U46619. Phosphorylation occurred rapidly and was inhibited when platelets were preincubated with the TXA2/PGH2 receptor antagonist, SQ29548 (5 microM), before being stimulated with U46619. These results suggest that human platelet TXA2/PGH2 receptors are phosphoproteins and that the level of phosphorylation is increased during homologous desensitization.
authors.affiliation: Department of Cell and Molecular Pharmacology, Medical University of South Carolina, Charleston 29425.
authors.forename: A K
authors.initials: AK
authors.lastname: Okwu
authors.forename: D E
authors.initials: DE
authors.lastname: Mais
authors.forename: P V
authors.initials: PV
authors.lastname: Halushka
mesh_terms: D019796:15-Hydroxy-11 alpha,9 alpha-(epoxymethano)prosta-5,13-dienoic Acid; D001792:Blood Platelets / Q000187:drug effects / Q000378:metabolism*; D019086:Bridged Bicyclo Compounds, Heterocyclic; D002462:Cell Membrane / Q000378:metabolism; D002846:Chromatography, Affinity; D005231:Fatty Acids, Unsaturated; D006801:Humans; D006834:Hydrazines / Q000494:pharmacology; D066298:In Vitro Techniques; D007700:Kinetics; D010710:Phosphates / Q000097:blood; D010761:Phosphorus Radioisotopes; D010766:Phosphorylation; D011450:Prostaglandin Endoperoxides, Synthetic / Q000494:pharmacology; D044262:Prostaglandin H2; D011463:Prostaglandins H / Q000097:blood; D011869:Radioligand Assay; D011982:Receptors, Prostaglandin / Q000187:drug effects / Q000302:isolation & purification / Q000378:metabolism*; D017482:Receptors, Thromboxane / Q000187:drug effects / Q000302:isolation & purification / Q000378:metabolism*; D044244:Receptors, Thromboxane A2, Prostaglandin H2; D013928:Thromboxane A2 / Q000031:analogs & derivatives / Q000037:antagonists & inhibitors / Q000494:pharmacology; D014662:Vasoconstrictor Agents / Q000494:pharmacology
chemical_list: D019086:Bridged Bicyclo Compounds, Heterocyclic; D005231:Fatty Acids, Unsaturated; D006834:Hydrazines; D010710:Phosphates; D010761:Phosphorus Radioisotopes; D011450:Prostaglandin Endoperoxides, Synthetic; D011463:Prostaglandins H; D011982:Receptors, Prostaglandin; D017482:Receptors, Thromboxane; D044244:Receptors, Thromboxane A2, Prostaglandin H2; D014662:Vasoconstrictor Agents; D044262:Prostaglandin H2; D013928:Thromboxane A2; D019796:15-Hydroxy-11 alpha,9 alpha-(epoxymethano)prosta-5,13-dienoic Acid; C045749:SQ 29548