Yangambin: A New Naturally-Occurring Platelet-Activating Factor Receptor Antagonist: Binding andIn VitroFunctional Studies

The effects of the furofuran lignan yangambin on rabbit platelet aggregation and binding of [3H]-PAF to rabbit platelet plasma membranes were studied. Log concentration-response curves to PAF were obtained in the presence or absence of increasing concentrations of yangambin. This lignan dose-dependently inhibited PAF-induced platelet aggregation in platelet-rich plasma (PRP) and shifted PAF curves to the right without decreasing the maximal response. The Schild plot constructed from these data showed a slope of 1.17 and a pA2 of 6.45. Moreover, yangambin at 10(-5) M did not inhibit the platelet aggregation induced by ADP (5 x 10(-7) M), collagen (0.1 microgram ml-1), or thrombin (0.05 U ml-1). Biochemical studies showed that [3H]-PAF labelled in a saturable manner a single class of binding sites on platelet membranes with a Kd of 1.25 +/- 0.24 nM and a maximal binding capacity (Bmax) of 14.9 +/- 2.4 pmol mg protein-1. Both unlabelled PAF and yangambin competitively displaced [3H]-PAF binding with an IC50 of 1.54 +/- 0.37 nM and 1.93 +/- 0.53 microM, respectively. The incubation of rabbit blood neutrophils with yangambin at 10(-5) M did not prevent PAF-induced in vitro chemotaxis in conditions where the PAF antagonist SR 27417 at 10(-5) M abolished the phenomenon. These results indicate that yangambin is an antagonist that selectively blocks PAF receptors on platelets.