Effects of platelet activating factor receptor antagonists on intracellular platelet activating factor function in...

We investigated the effects of the platelet activating factor (PAF) receptor antagonists, SM-12502 ((+)-cis-3,5-dimethyl-2-(pyridyl)- thiazolidin-4-one hydrochloride), WEB-2086 (3-(4-(2-chlorphenyl)-9-methyl-6H-thieno(3,2-f)-(1,2,4)triazolo(4, 3- a)(1,4)diazepin-2-yl)-1-(4-morpholinyl)-1-propanone) and RP-48740 (3-(3-pyridyl)-1H,3H-pyrrolo[1,2-c]thiazole-7-carboxamide) on the PAF-mediated activation of rat neutrophils. These antagonists inhibited PAF-induced degranulation and chemotaxis in neutrophils at a dose that correlated well with PAF-induced platelet aggregation based on the statistical analyses. N-formyl-L-methionyl-L-leucyl-L- phenylalanin (fMLP)-induced cellular responses were also inhibited by the PAF receptor antagonists, but their inhibitory potencies did not correlate with those for PAF-induced platelet aggregation. In addition, the doses required for inhibition were higher than those required against PAF-induced responses (i.e. IC50 ratio of WEB-2086, SM-12502 and RP-48740 in fMLP-induced/PAF-induced degranulation was 40.0, 2.8 and 5.6, respectively). PAF receptor antagonists inhibited inositol 1,4,5-triphosphate production and the release of Ca2+ from the intracellular store site after stimulation with PAF. In the fMLP-induced responses, PAF receptor antagonists did not inhibit IP3 production and Ca2+ release, but did inhibit transmembrane Ca2+ influx. These results suggest the presence of distinct PAF receptor subtype, to which exogenously added PAF binds, while endogenously produced PAF binds to the other. Intracellular PAF, which was produced by fMLP-stimulation, may play an important role in the late phase of signal transduction, and may participate in the transmembrane Ca2+ influx.