Action of 5-hydroxytryptamine on the human stomach, duodenum, and jejunum in vitro.

Abstract
Responses to acetylcholine (ACh) serotonin (5-HT) were studied with isolated strips in a modified Krebs'' solution at 37[degree]C. Longitudinal muscle of the body of the stomach regularly contracted in response to 5-HT. Circular gastric muscle and both layers of muscle in the antrum and first part of the duodenum did not respond to 5-HT, although they responded to ACh. Both muscle layers of the proximal jejunum were very sensitive to 5-HT. Atropine blocked responses to ACh but not to 5-HT. Lysergic acid diethylamide blocked 5-HT but not ACh. Procaine had no effect on ACh or 5-HT responses. 5-HT, when it causes contraction, appears to do so by a direct action on the muscle. The oblique layer of muscles in the stomach was not studied.