Inhibition of platelet-activating factor binding to human platelets by calcium channel blockers

Calcium channel blockers may impair cell activation either by inhibiting calcium influx or by inhibiting agonist binding. Because of this dual action of calcium channel blockers and because of the close relationship between calcium influx and platelet-activating factor (PAF) binding to platelets the current studies examined the effect of calcium channel blockers on PAF binding to washed human platelets. Diltiazem and verapamil inhibited aggregation by PAF in a dose-dependent manner with 50% inhibition at 2.8 +/- 1.4 X 10(-5) M diltiazem (mean +/- SD, n = 5) and 4.2 +/- 2.0 X 10(-5) M verapamil. Both channel blockers also inhibited PAF binding in a dose-dependent manner with 50% inhibition at 4.7 +/- 2.5 X 10(-5) M diltiazem and 6.3 +/- 1.2 X 10(-5) M verapamil. Analysis of the mechanisms of inhibition of binding indicate both competitive and non-competitive effects of the channel blockers. Scatchard analysis of PAF binding in the presence of different fixed concentrations of either diltiazem or verapamil revealed that these agents both increased PAF receptor number and decreased the receptor binding affinity. Lineweaver-Burke analysis of the same data revealed a family of lines which intersect to the right of the ordinate. The channel blockers also dissociated previously-bound PAF from platelets. The current studies indicate that calcium channel blockers inhibit platelet activation by PAF by more than one mechanism and suggest that the PAF receptor may be closely associated with calcium channels.