title: Polymyxin B increases TXA2/PGH2 agonist and antagonist affinities in human platelet membranes.
abstract: The effects of polymyxin B, a cyclic polycationic peptide antibiotic, on the binding of thromboxane A2/prostaglandin H2 (TXA2/PGH2) receptor agonists and antagonists to the putative human platelet TXA2/PGH2 receptor have been determined. Crude membranes were prepared from washed human platelets and the binding of [125I]-PTA-OH, a TXA2/PGH2 receptor antagonist, was used as a ligand to characterize the effects of polymyxin B on agonist and antagonist binding. Polymyxin B produced a concentration-dependent decrease in the IC50 values for the TXA2/PGH2 agonist, U46619, in radioligand competition binding assays. Polymyxin B also decreased the IC50 values of the TXA2/PGH2 agonist ONO 11113 from 75 +/- 11 nM to 38 +/- 2.5 nM and the TXA2/PGH2 receptor antagonist SQ29548 from 60 +/- 14 nM to 20 +/- 2.3 nM. The Kd for [125I]-PTA-OH was also reduced by polymyxin B from 107 +/- 10 nM to 76 +/- 6 nM but it had no effect on the maximum number of binding sites (Bmax). Polymyxin B had no significant effect on the binding affinity of U46619 in intact washed human platelets. These results raise the possibility that polymyxin B can alter the conformational state of the TXA2/PGH2 receptor when it is in a crude membrane fraction but not in intact platelets.
authors.affiliation: Department of Cell and Molecular Pharmacology and Experimental Therapeutics, Medical University of South Carolina, Charleston 29425.
authors.forename: P V
authors.initials: PV
authors.lastname: Halushka
authors.forename: A
authors.initials: A
authors.lastname: Boehm
authors.forename: D E
authors.initials: DE
authors.lastname: Mais
mesh_terms: D019307:1-(5-Isoquinolinesulfonyl)-2-Methylpiperazine; D019796:15-Hydroxy-11 alpha,9 alpha-(epoxymethano)prosta-5,13-dienoic Acid; D001792:Blood Platelets / Q000187:drug effects* / Q000378:metabolism / Q000648:ultrastructure; D019086:Bridged Bicyclo Compounds, Heterocyclic; D002462:Cell Membrane / Q000187:drug effects / Q000378:metabolism; D005231:Fatty Acids, Unsaturated; D006801:Humans; D006834:Hydrazines / Q000378:metabolism; D066298:In Vitro Techniques; D007546:Isoquinolines / Q000494:pharmacology; D010879:Piperazines / Q000494:pharmacology; D011112:Polymyxin B / Q000494:pharmacology*; D011113:Polymyxins / Q000494:pharmacology*; D011449:Prostaglandin Endoperoxides / Q000378:metabolism*; D011450:Prostaglandin Endoperoxides, Synthetic / Q000378:metabolism; D011463:Prostaglandins H / Q000378:metabolism*; D047428:Protein Kinase Inhibitors; D011982:Receptors, Prostaglandin / Q000187:drug effects* / Q000378:metabolism; D017482:Receptors, Thromboxane; D044244:Receptors, Thromboxane A2, Prostaglandin H2; D013928:Thromboxane A2 / Q000031:analogs & derivatives / Q000378:metabolism*
chemical_list: D019086:Bridged Bicyclo Compounds, Heterocyclic; D005231:Fatty Acids, Unsaturated; D006834:Hydrazines; D007546:Isoquinolines; D010879:Piperazines; D011113:Polymyxins; D011449:Prostaglandin Endoperoxides; D011450:Prostaglandin Endoperoxides, Synthetic; D011463:Prostaglandins H; D047428:Protein Kinase Inhibitors; D011982:Receptors, Prostaglandin; D017482:Receptors, Thromboxane; D044244:Receptors, Thromboxane A2, Prostaglandin H2; D013928:Thromboxane A2; D019796:15-Hydroxy-11 alpha,9 alpha-(epoxymethano)prosta-5,13-dienoic Acid; C037866:STA 2; D019307:1-(5-Isoquinolinesulfonyl)-2-Methylpiperazine; C045749:SQ 29548; C046957:I-PTA-OH; D011112:Polymyxin B