Histamine potentiates human platelet aggregation induced by platelet-activating factor

Using human platelet-rich plasma (PRP) we found that histamine concentration-dependently potentiated platelet aggregation induced by platelet-activating factor (PAF). When added separately, neither histamine nor chloropyramine, a competitive antagonist of H1 receptors, influenced platelet aggregation, however BN 52021, a specific PAF receptor antagonist, concentration-dependently inhibited the response. Chloropyramine reduced the potentiating effect of histamine on PAF-induced platelet aggregation, and when it was coadministered with BN 52021, inducing 63% inhibition of PAF-induced aggregation, the histamine-potentiated PAF response was completely abolished. This observation draws attention to the importance of cooperation between histamine and PAF in mediation of platelet aggregation in human PRP, and points to the powerful antiaggregatory effect of BN 52021 combined with an antagonist of H1 receptors.