Excitatory action of sympathomimetic amines on 5-hydroxytryptamine receptors of gut.

1. Twenty-two sympathomimetic amines were tested for excitatory activity on isolated guinea-pig ileum, rabbit jejunum, and rat stomach.2. Eight amines contracted all or almost all guinea-pig ileum preparations; five amines contracted half the preparations and relaxed the others. Nine amines were consistently relaxant. Structural requirements for excitatory activity were not clear.3. Six amines contracted rat stomach and only four contracted rabbit jejunum. Only beta-phenylethylamine and (-)-amphetamine contracted all three preparations.4. The excitatory effects of the sympathomimetic amines on guinea-pig ileum seem to be due to an action on 5-hydroxytryptamine receptors. This conclusion is based on evidence that their excitatory action is antagonized by 2-bromolysergic acid diethylamide, by morphine and by desensitization with 5-hydroxytryptamine, and that there is cross-protection between 5-hydroxytryptamine and the excitatory sympathomimetic amines against block by phenoxybenzamine.