Dog platelets fail to aggregate when they form aggregating substances upon stimulation with arachidonic acid.

Dog platelets are refractory to aggregation by arachidonic acid (AA) but generate an unstable activity that aggregates rabbit platelets. Formation of this activity is inhibited by indomethacin, by the peroxide scavenging enzyme catalase, by two chelating agents that bind Cu+ and Cu2+ ions, by the -SH agent dithiothreitol and is stimulated by cysteine. Agitation of dog platelets is followed by spontaneous aggregation and uncovers aggregation by AA, which is blocked by indomethacin. Neither indomethacin nor apyrase prevent spontaneous aggregation, ruling out both activation of prostaglandin synthetase and leakage of ADP as possible explanations. Complexation of plasma Ca2+ by citrate as an explanation for refractoriness to AA was ruled out by replacing citrate with heparin. Dog platelets are also refractory to PGH2 formed from AA by the cyclo oxygenase component of prostaglandin synthetase. Aggregation of rabbit platelets by PGH2 is not inhibited by indomethacin, by catalase, by dithiothreitol or by metal chelating agents and is not potentiated by cysteine. This confirms that the reagents act before PGH2 is formed. Aggregating activity generated by dog platelets is probably due to an unstable lipoperoxide whose generation involves mechanisms similar to those responsible for aggregation of rabbit platelets, since similar antagonists block both processes.