Stable synthetic mimetics of thromboxane (TX) A2 and prostaglandin (PG) H2 have been synthesized and reported to stimulate platelets and vascular smooth muscle. The synthetic agonists induce aggregation of isolated platelets and contraction of vascular tissue. The tritiated agonists [3H]U46619 and [3H]U44069 have been used in radioligand binding studies to characterize platelet and vascular smooth muscle TXA2/PGH2 receptors, but have limited usefulness due to their low specific activities and variable specific binding. In an attempt to overcome these problems, we have synthesized a stable, high affinity, 125I-radiolabeled TXA2/PGH2 receptor agonist, [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-heptenoic acid (I-BOP). I-BOP induced shape change, increased intracellular free calcium concentrations and aggregated isolated human platelets (EC50 = 0.21 +/- 0.05 nM, n = 3; 4.1 +/- 1.1 nM, n = 4; 10.8 +/- 3 nM, n = 9, respectively). The kinetically determined Kd was 1.02 +/- 0.33 nM (kobs = 0.19 +/- 0.05 min-1, k-1 = 0.097 +/- 0.02 min-1, k1 = 0.119 +/- 0.03 min-1 M, n = 4). Equilibrium binding studies of [125I]BOP to isolated human platelets indicated one class of high affinity sites, Kd = 2.2 +/- 0.3 nM and a maximum binding of 0.028 +/- 0.002 x 10(-12) mol/10(7) platelets (1699 +/- 162 sites/platelet, n = 9).(ABSTRACT TRUNCATED AT 250 WORDS)