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SummaryThe activity of several synthetic compounds, rated from good to poor (or inactive) fibrinolytic activators, has been assessed by two different commonly-used in vitro methods. Compounds shown to be active over a narrow concentration range in the hanging clot test were shown to be inhibitors of plasmin and trypsin in the casein-olytic test. The inhibitory activity of these compounds was shown to increase with increasing substrate concentration and apparent activity in the hanging clot test. Possible explanations and relevance of these observations are discussed.

SummaryThe activation characteristics of the fibrinolytic system of several species have been evaluated in vitro and compared with the human enzyme system. The cat, dog and baboon would appear to be the most suitable species for further study of fibrinolytic activation. The present findings have been found to parallel the species differences in platelet behaviour which have been reported separately from this laboratory.

SummaryWhen some human platelets were aggregated in vitro by exogenous adenosine diphosphate (ADP) or adrenaline they released their own ADP. As ADP is the final stimulator of most aggregation, this ability to produce its own release gave a self- propagating secondary aggregation (SA) which could be marked in certain individuals and diseased states. In a survey of several animals, this ability to produce self- propagating aggregation was also found in a marked form in cats (which can form systemic platelet thrombo-emboli) and to a less extent in baboons and some dogs. This property of the platelets of humans and cats could be an important factor in their ability to form platelet thrombi more easily than other mammals. Thus, SA and its inhibition by drugs may be of considerable importance in the management of thromboembolic disease and a study of such agents carried out in these animals may be reasonably regarded as a relevant and useful preliminary to clinical trials in man.