Aggregation of Blood Cells by 5-Hydroxytryptamine (Serotonin)

Abstract

Addition of small amounts of 5-hydroxytryptamine to whole blood altered the tendency of the platelets, leucocytes, and red blood cells of dogs, cats, and rabbits to aggregate. The magnitude of this response to 5-hydroxy-tryptamine was dose dependent and biphasic. Very small doses seemed to decrease the aggregation tendency; larger amounts greatly increased this tendency. This change was determined by measuring the pressure required to force blood at a constant speed through a standardized screen with multiple pores 20 micra square. This pressure was referred to as the "screen filtration pressure." The in vitro serotonin effect was abolished by very slight reductions of pH, by addition of KCN to blood, and by removing adhesive platelets from the blood. In vivo 5-hydroxytryptamine caused aggregation of blood elements resulting in obstruction of blood flow in many blood vessels of the conjunctiva. This was accompanied by a reduction in hematocrit when multiple injections of 5-hydrosytryptamine were given. In a few dogs the addition of serotonin to blood in vitro did not increase the screen filtration pressure. In these, adenosine diphos-phate (ADP) caused a slight increase in this pressure, and together with serotonin caused a very marked increase in the screen filtration pressure. Serotonin inhibitors inhibited the effects of 5-hydroxytryptamine, ADP, and combinations of ADP and 5-hydroxytryptamine.