Normal plasma fraction having anti-platelet aggregation activity

A man (66 years old) having plasma which showed an inhibitory activity of platelet aggregation induced by ADP, thrombin, epinephrine or collagen was found by Funahara, et al. Other abnormal findings observed were an increase of blood platelets number (106/μl) and a loss of epinephrine induced aggregation activity of platelets. It was uncertain whether the inhibitor was newly formed as a result of thrombocythemia or an increase of the inhibitor production was accompanied by thrombocythemia. In order to clarify this question, an existence of the anti-platelet aggregating activity in plasma from healthy subjects was examined by the following experiments as a first step. The study of change of free platelets number in platelet suspension medium by Coulter-Counter ZBI showed that an activity to block platelet aggregation by ADP, thrombin or epinephrine existed in normal plasma. Though this activity was shown to reside in the outer fluid of bag by plasma dialysis, it was observed in void volume fraction when the outer fluid was passed through Sephadex G10 column, showing molecular weight of the inhibitor was small but somewhat lager than about 400 molecular weight. The activity was not affected by the treatment of plasma in boiling water for 20 minutes. Changes of the activity were not observed in the presence of aspirin, suggesting that the inhibiting site of the plasma inhibitor did not coincide with that of aspirin. The inhibitor showing very similar property to ours was reported by Mason, et al. (Am. J. Path. 74: 91a, 1974), but our inhibitor seems to be different from theirs in the behavior in Sephadex G10 column.