Enterochromaffin Cells And Gastrointestinal Tract

Enterochromaffin cells of the GI mucosa (highest density in the duodenum) synthesize and store 5-HT and other autacoids. Basal release of enteric 5-HT is augmented by mechanical stretching and efferent vagal stimulation. 5-HT probably has an additional role in stimulating motility via the myenteric network of neurons (see Chapters 6 and 37). 5-HT released from enterochromaffin cells enters the portal vein and is subsequently metabolized by MAO-A in the liver. 5-HT that survives hepatic oxidation is rapidly removed by the endothelium of lung capillaries and then inactivated by MAO. 5-HT released by mechanical or vagal stimulation also acts locally to regulate GI function. Motility of gastric and intestinal smooth muscle may be either enhanced or inhibited via at least six subtypes of 5-HT receptors (Table 11-2). Abundant 5-HT3 receptors on vagal and other afferent neurons and on enterochromaffin cells play a pivotal role in emesis (see Chapter 37). Enteric 5-HT is released in response to ACh, sympathetic nerve stimulation, increases in intralu-minal pressure, and lowered pH, triggering peristaltic contraction.

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