Aggregation of Blood Cells by 5-hydroxytryptamine (serotonin).

• A rapid, reproducible method of determiniEg the presence and relative amounts of adhesive and aggregated blood elements in the blood has recently been developed in this laboratory.The method involves measuring the pressure required to force blood at a constant rate through a standardized screen with multiple openings 20 micra square and 20 micra deep. This pressure has been named "screen nitration pressure." During storage of blood and during exsanguination, platelets and leucocytes form aggregates which cause remarkable increases in this pressure. It has also been observed that adenosine diphosphate; a component of platelets, causes platelets suspended in plasma to aggregate, and that monoamine oxidase inhibitors inhibit platelet aggregations. These observations have led to an interest in the composition of the platelets, and to further studies of the effects of certain components of platelets on the suspension stability of blood. One of these constituents, 5-hydroxytryptamine (serotonin), has become particularly interesting in this respect.