Characterization of thromboxane A2/prostaglandin H2 binding sites in guinea pig cardiac membrane preparations.

Thromboxane A2 (TXA2) and prostaglandin H2 (PGH2) induce platelet aggregation and are potent vasoconstrictors, and they have been implicated in coronary vasospasm and myocardial infarction. The TXA2 mimetic [1S-(1 alpha, 2 beta (5Z), 3 alpha (1E,3S*), 4 alpha)]-7-[3-(3-hydroxy-4-(4'- iodophenoxy)-1-butenyl)-7-oxabicyclo-[2.2.1]heptan-2-yl]-5-h eptenoic acid (IBOP) was used to characterize binding to microsomal membrane preparations from saline-perfused guinea pig atria (GPA) and ventricles (GPV). [125I]IBOP bound to GPA and GPV in a protein-dependent and saturable manner, although total binding was two-fold greater and non-specific binding was proportionately less in GPA compared to GPV. Analysis of equilibrium binding data indicated one class of binding sites in both GPA and GPV with Kd values of 333 +/- 117 and 645 +/- 187 pM, respectively, which were in close agreement with kinetically determined Kd values of 226 and 882 pM, respectively. Bmax values of GPA and GPV of 57 +/- 5.6 and 24 +/- 4.3 fmol/mg protein were significantly different (P < 0.01). Ki values (from IC50s) were determined for various TXA2/PGH2 analogues and prostaglandins in competition binding assays with [125I]IBOP. The rank order for ability to inhibit binding in GPA was U46619 = SQ29548 > I-PTA-OH > PGF2 alpha = PGE2. In GPV, the rank order was U46619 = SQ29548 > PGF2 alpha = I-PTA-OH = PGE2. [125I]IBOP binding to GPA and GPV was completely displaced by the TXA2/PGH2 agonist U46619 and by the TXA2/PGH2 antagonist SQ29548.(ABSTRACT TRUNCATED AT 250 WORDS)