Interaction of the Paf antagonist WEB 2086 and its hetrazepine analogues with human platelets and endothelial cells.

1. Intact platelets and confluent human umbilical vein endothelial cells bound [3H]-Paf-acether (platelet activating factor, [3H]-Paf) at 20 degrees C in the presence of 0.25% (w/v) bovine serum albumin (BSA). 2. [3H]-Paf binding to platelets was inhibited in a concentration-dependent manner by WEB 2086. An excess of WEB 2086 indicated the presence of specific, saturable Paf binding which reached a maximum of 28.3 +/- 3.7 fmol [3H]-Paf per 5 x 10(7) platelets. In platelets, different hetrazepines (WEB 2098, 2105, but not 2118) also inhibited [3H]-Paf binding in a concentration-dependent manner. 3. WEB 2086 partially displaced platelet-bound [3H]-Paf in a concentration-dependent manner reaching a plateau at 400 nM WEB 2086. No further displacement was observed when WEB 2086 and an excess of unlabelled Paf were added together. 4. The hetrazepines inhibited platelet aggregation. Platelet aggregation IC50 values correlated well with the IC50 values of the hetrazepines against [3H]-Paf binding (r2 = 0.99). WEB 2086 shifted the Paf dose-response curve rightwards in a parallel manner. Tested against platelet aggregation the pA2 obtained for WEB 2086 was 7.9. 5. WEB 2086 inhibited [3H]-Paf binding to endothelial cells in a concentration-dependent manner. WEB 2086 also inhibited the Paf-mediated cytosolic calcium increase in endothelial cells with an IC50 value of 23.1 +/- 10.4 nM as compared with an IC50 of 21.6 +/- 10.4 nM WEB 2086 for platelet aggregation. 6. These results demonstrate an inhibition of [3H]-Paf binding to platelets and endothelial cells by different hetrazepines, most probably at the Paf receptor level.