Specific binding sites of platelet activating factor on the intact bovine cerebral microvascular endothelial cells and antagonism of drugs

[3H]Triazolodiazepine ([3H]WEB 2086), an antagonist of platelet activating factor (PAF) receptor, was studied as radioligand on intact cerebral microvascular endothelial cells (CMEC). The results showed that the binding of [3H]triazolodiazepine reached and maintained at an equilibrium after 15-120 min of incubation and that it was saturable with increasing concentration of radioligand. Scatchard analysis indicated that there were 2 specific binding sites on CMEC, its Kd1, Bmax1, Kd2, and Bmax2 were 3.13 nmol.L-1, 1.50 pmol/3 x 10(5) cells, 83.96 nmol.L-1, and 12.96 pmol/3 x 10(5) cells, respectively. The binding of [3H]triazolodiazepine to CMEC was displaced by C16-PAF and 1,5-bis-(3,4-dimethoxyphenyl)-tetrahydro-(4H)-pyran (SZ-1), which IC50 were 0.43 nmol.L-1 and 0.125 mumol.L-1, respectively. These data suggested the existence of PAF specific binding sites on CMEC.