1. Using high-performance liquid chromatography with electrochemical detection, we have studied the release of endogenous catecholamines from washed platelets induced to aggregate by ADP and thrombin.

2. Washed platelets exhibit irreversible aggregation with 10 μmol/l ADP and 0.3 unit of thrombin/ml, extents of aggregation being 27% and 76% respectively.

3. ADP (10μmol/l) increased noradrenaline release to the medium by 20% and adrenaline by 28%. As observed with aggregation, 0.3 unit of thrombin/ml produced a more marked effect on release than ADP, noradrenaline and adrenaline being increased by 570% and 169% respectively.

4. Values for platelet noradrenaline content were found to mirror those for the release from platelets induced by thrombin.

5. A correlation was observed between catecholamine release and the concentration of thrombin present in incubations. These data reflect the changes observed with platelet aggregation.

6. This is the first study to determine catecholamine release from platelets by direct measurement. Local catecholamine release after platelet aggregation in vivo may have important consequences for tissue perfusion.

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