RELATION BETWEEN 5-HYDROXYTRYPTAMINE CONTENT AND AGGREGATION OF RABBIT PLATELETS

1. Blood platelets containing different amounts of 5‐hydroxytryptamine (5–HT) were produced in vivo by the injection of 5‐hydroxytryptamine or of reserpine into normal rabbits and of 5‐hydroxytryptamine into reserpinized rabbits. Before and after these injections the aggregation of platelets was measured in vitro. 2. Platelets of untreated rabbits were aggregated by adenosine diphosphate (ADP) and by 5‐hydroxytryptamine; (—)‐adrenaline alone did not produce aggregation but markedly increased aggregation by 5‐hydroxytryptamine. 3. Platelets saturated with 5‐hydroxytryptamine in vivo were no longer aggregated in vitro by 5‐hydroxytryptamine or by 5‐hydroxytryptamine plus adrenaline, but their aggregation by ADP was unchanged. 4. Platelets from reserpinized rabbits lost about 99% of their 5‐hydroxytryptamine; the aggregation of these platelets did not differ significantly from that of platelets from control rabbits. 5. Platelets from reserpinized rabbits injected with 5‐hydroxytryptamine were aggregated neither by the amine alone nor by 5‐hydroxytryptamine plus adrenaline, although these platelets contained much less 5‐hydroxytryptamine than saturated platelets and only about one tenth as much as platelets from untreated rabbits. 6. The findings support the hypothesis that the inhibitory effect of 5‐hydroxytryptamine administered in vivo on platelet aggregation in vitro is due to the association of the amine with the platelet membrane. © 1969 The Physiological Society