The 5-hydroxytryptamine-like actions of 5,6-dihydroxytryptamine.

1. With isolated preparations of rat stomach fundus as well as of duodenum and ileum of rats and guinea-pigs, 5,6-dihydroxytryptamine and 5,6-diacetoxytryptamine caused a contraction which was antagonized by methysergide and lysergic acid diethylamide (LSD), but not by atropine. Pretreatment of the animals with reserpine did not decrease the effect of the two indoleamines on the isolated ileum and duodenum.2. In anaesthetized guinea-pigs, 5,6-dihydroxytryptamine and its diacetyl derivative caused bronchoconstriction which was antagonized by methysergide, but not modified by pretreating the animals with reserpine.3. In anaesthetized cats, 5,6-dihydroxytryptamine had, in general, a hypotensive effect which was reversed by hexamethonium.4. 5,6-Dihydroxytryptamine also caused aggregation of isolated rabbit and human platelets and inhibited the platelet aggregation induced by 5-hydroxytryptamine (5-HT) plus adrenaline.5. The pattern of action of 5,6-hydroxytryptamine and 5,6-diacetoxytryptamine was qualitatively the same as that of 5-HT, but the potency of the compounds decreased in the order 5-HT, 5,6-dihydroxytryptamine, 5,6-diacetoxytryptamine both in vitro and in vivo.6. It is concluded that 5,6-dihydroxytryptamine and its diacetyl derivative stimulate postsynaptic 5-HT receptors, but that their effect is weaker than that of 5-HT.