Inhibition by CV-3988 of the binding of [ 3H]-platelet activating factor (PAF) to the platelet

Abstract

The inhibitory effects of CV-3988, a specific antagonist of PAF, on the binding of [ 3H]-PAF to washed platelets of various species including human were examined. The dissociation constant ( K d), binding capacity ( B max), and the number of receptor/platelet for the specific binding site of rabbit platelets were 2.2 ± 0.2 nM, 93.7 ± 8.3 fmoles/10 8 platelets, and 568 ± 50, respectively. CV-3988 selectively inhibited the specific binding of [ 3H]-PAF to rabbit platelets with an IC 50 of 7.9 × 10 −8 M, and it slightly increased the K d value (2.5 ± 0.8 nM) and decreased the binding capacity for PAF ( B max: 54.3 ± 16.3 fmoles/10 8 platelets). The K i value of CV-3988 for the specific binding of [ 3H]-PAF to rabbit platelets was 1.2 × 10 −7 M. CV-3988 had no effects on the binding of [ 3H]-5-hydroxytryptamine (5-HT) to rabbit platelets and on the shaoe change of the platelet induced by 5-HT. CV-3988 also inhibited the specific binding of [ 3H]-PAF to human and guinea-pig platelets with IC 50 values of 1.6 × 10 −7 and 1.8 × 10 −7 M, respectively. CV-3988 inhibited the PAF-induced aggregation in rabbit, guinea-pig, and human platelets. These findings show that CV-3988 is a specific antagonist of PAF at the receptor site(s) of platelets and, in these species, inhibits PAF-induced platelet aggregation by inhibiting the binding of PAF to the “PAF receptor”. No specific binding of [ 3H]-PAF to the platelet of rats and mice was observed, indicating that these species lack a PAF receptor.