Polymyxin B increases TXA2/PGH2 agonist and antagonist affinities in human platelet membranes

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.