Effect on human platelets of catecholamines at levels achieved in the circulation.

Epinephrine at concentrations varying between 3.3 and 12.5 nM had no effect on blood platelets when added alone, but augmented the in vitro platelet response to collagen and thrombin. Both aggregation and secretion responses were enhanced. Norepinephrine produced similar effects but was 50-60% less active than epinephrine. The minimum concentration of epinephrine or norepinephrine to achieve potentiation of platelet responses was even lower in the presence of 5-hydroxy- tryptamine. In contrast to the effects observed with higher concentrations of catecholamines, the synergistic interaction of these low concentrations of catecholamines with other agonists was not transient. The augmented response to catecholamines was mediated by platelet alpha 2-adrenoceptors. The response was inhibited by aspirin indicating that metabolism of arachidonic acid contributes to the synergy between low concentrations of catecholamines and other agonists. These studies show that the levels of the hormones epinephrine and norepinephrine obtained in circulating blood in humans, can be sufficient to enhance platelet responses. The action of catecholamines on platelets may be important in hemostasis and could provide an explanation for the association between certain risk factors and cardiovascular disease.