The effect of Dipyridamole on platelet adhesiveness and aggregation

Abstract

The effect of dipyridamole on human platelet adhesivenss was studied by the author's modification of Hellem's II method and its effect on platelet aggregation was observed by the turbidmetric method of Born using aggregometer (Bryston).The adhesivenss, in vitro, was inhibited by dipyridamole at 100-500μg/ml blood, and the inhibited degree of the the adhesiveness was slighter than that of ADP-induced aggregation. In vivo, by intravenous injection of dipyridamole, the adhesiveness was inhibited obviously during 5-15 minutes, and recovered after 30 minutes. It was also lowered during the course of oral abministration of dipyridamole (75-250mg/day), but was rised by the nterruption of its administration.ADP-aggregation (0.4μg) was also inhibited by dipyridamole at 50-500μg/ml PRP. Only secondary aggregation was inhibited by its low concentrations, whereas primary aggregation was also inhibited by its high concentrations. Lag phases of adrenalin (1μg)-and collagen (50%)-aggregation were prolonged by this inhibitor at 50-200μg/ml, and such aggregation was completely inhibited at the concentration, over 200μg/ml.The inhibitory effect of dipyridamole upon ADP-induced aggregation was not influenced by the period of its pre-incubation with PRP and by HCl used in these experiments. Platelet aggregation induced by ADP (0.4-40μg/ml of PRP) which had been incubated with 100μg of dipyridamole was similar to that induced by the same concentrations of ADP without the preincubation.Fifty μg of dipyridamole/ml PRP slightly inhibited ADP release induced by Kaolin, and its concentration over 200μg/ml inhibited the release so remarkably that ADP thus released was not enough to aggregate platelets.Bovine fibrinogen-aggregation was not influenved by dipyridamole at the concentration up to 500μg/ml.