In the guinea-pig, platelet aggregation by several agonists results in a bronchoconstriction. Thus we speculated that SRS could be the mediator for this phenomenon, eventhough it has not been yet described as originating from glatelets.Washed rabbit platelets (2.5 × 108) stimulated by plateletactivating factor (PAF-acether) (0.72 nM), thrombin (0.15 unit/ml) or arachidonic acid (AA) (10 uM) released 2.7 ± 0.9, 2.5 ± 0.2 and 1.4 ± 0.8 units of SRS, respectively (5 exp., 1 unit = guinea-pig ileum contraction induced by 5 ng histamine). By contrast, ADP (10-100 uM) aggregated platelets but did not induce the release of SRS. Preincubation of platelets with aspirin (0.1 mM : 10 min), or the presence of indomethacin (5 uM) in the incubating media, did not modify the yield of SRS, thus excluding prostaglandins as the contracting substance. SRS release was not correlated with the aggregation since aspirin-treated platelets failed to aggregate in the presence of AA but still released SRS, whereas untreated platelets ADP-stimulated, did aggregate without releasing SRS.L-cysteine (20 mM) increased 10 fold the yield of SRS and eicosatetraynoic acid (ETYA) (10-100 uM) caused a dose-dependent inhibition of SRS release.Platelet SRS was resistant to base but not to acid hydrolysis and its action on guinea-pig ileum was reversed by FPL 55712 (1 ug/ml). It was further purified on reverse phase high pressure liquid chromatography (RP-HPLC) using a discontinuous gradient of methanol in water. Platelet-SRS was eluted as 3 peaks of activity (retention times : 39 min, 46 min, 54 min) as did SRS from ionophore-stimulated rat peritoneal cells. Synthetic leukotrienes C and D exhibited retention times of 46 min and 54. min respectively. SRS obtained from platelets preincubated with 14C- AA was also analyzed on RP-HPLC. Only one peak of activity (54 min) coincided with radioactivity suggesting the incorporation of AA in this SRS molecule.These data bring evidence for the release from stimulated rabbit platelets of a SRS closely related to leukotrienes.
Inhibitory effects of paraquat on guinea pig ileum contraction
