Histamine release from rabbit platelets by platelet-activating factor (PAF).

: In this study, we examined firstly whether PAF could activate washed platelets of rabbits and release ATP from the platelets, secondly whether activated platelets could release histamine, and thirdly whether certain antagonists of PAF could inhibit the release of histamine from platelets. The results obtained in these experiments may be summarized as follows. 1) Release of ATP was increased with enhanced platelet aggregation by PAF. At 3.4 x 10(-7) M of PAF, the maximal aggregation rate of platelets was about 70% and the concentration of released ATP was 1.6 x 10(-5) M. 2) After aggregated platelets induced by PAF had been sonicated and centrifuged, the resultant supernatant could cause contraction of the guinea pig ileum. This contraction was inhibited by the antihistaminic agents, cimetidine and pyrilamine. Furthermore, the histamine content of the supernatant was about 3.7 micrograms/ml (platelet count, 30 x 10(4)/microliter). 3) At 10(-5) M of CV-3988, the inhibitory ratio of PAF-induced aggregation was 35% and that of histamine release was 50%. On the other hand, at 5 x 10(-7) M of CV-6209, the inhibitory ratio of PAF-induced aggregation was 35% and that of histamine release was about 30%. From the above results, it was clear that PAF-activated platelets could release histamine. In addition, it is suggested that a direct relationship between PAF and platelets may exist in the process of allergic reactions, and histamine from PAF-activated platelets may modify allergic reactions.