Binding of a novel radioiodinated thromboxane A2/prostaglandin H2 antagonist to guinea pig lung membranes

The utilization of a novel radioiodinated TXA2/PGH2 receptor antagonist, ISAP (7-[(1R,2S,3S,5R)-6,6-dimethyl-3(4- iodobenzenesulfonylamino)-bicyclo[3.1.1]-hept-2-yl]-5(Z)-heptenoic acid) to characterize TXA2/PGH2 receptors from guinea pig lung parenchymal membranes in radioligand binding assays is described. [125I]ISAP binding was saturable, displaceable, and dependent upon protein concentration. The time course of binding yielded k1 = 2.12 x 10(8) M-1 min-1, k1 = 4.46 x 10(-3) min-1, Kd = k-1/k1 = 17.8 pM. Equilibrium binding studies indicated a single class of high affinity binding sites with a Kd of 52.7 +/- 1.9 pM and a Bmax of 92.7 +/- 7.2 fmoles/mg protein (n = 4). Binding was inhibited by a series of structurally diverse mimetics and antagonists with the rank order of potency IBOP greater than ONO11113 = SQ26655 greater than U46619 (mimetics) and (d)-S-145 greater than ISAP greater than (1)-S-145 greater than SQ29548 greater than BM13505 = I-PTA-OH (antagonists), with entantioselectivity of binding demonstrated by (d) and (1) S-145. Binding was also inhibited by prostanoids (PGD2, PGF2 alpha, and 9 alpha, 11 beta-PGF2) thought to act at the airway TXA2/PHH2 receptor, but not by histamine or carbachol, and only weakly by LTB4 and LTD4, consistent with specific binding to the lung TXA2/PGH2 receptor.