Serotonin and Carcinoid Tumors

: The enterochromaffin (EC) cell system is distributed throughout the entire gastrointestinal tract. Enterochromaffin cells are the major source of intestinal serotonin (5-HT), but separate subpopulations of EC cells may synthesize and store peptides as substance P (SP), motilin, and enkephalin as well. Of special interest is that 5-HT and SP, which may coexist in EC cells, have several functional similarities, i.e., inhibition of gastric acid secretion, stimulation of intestinal motility, and secretion of water and electrolytes. Carcinoid tumors are derived from the gut endocrine system. Depending on site of origin, carcinoids are divided into foregut, midgut, and hindgut derivatives with different clinical symptoms. A common biochemical feature of midgut carcinoids is the production of 5-HT and SP. Histochemically, midgut carcinoids are characterized by the argentaffin reaction--a direct reduction of silver salts owing to 5-HT. Specific antisera for the immunocytochemical demonstration of secretory products are available as well. Despite their relative infrequency, carcinoids are the most common small intestinal tumors. The common appendix tumors generally have a benign clinical course, whereas the small intestinal tumors have different growth patterns and frequently metastasize with increasing size, and may thus give rise to the carcinoid syndrome (diarrhea, facial flush, right-sided cardiac valvular disease, and asthma). Carcinoid symptoms first appear when hepatic inactivation of 5-HT is exceeded, unless the carcinoid has an extraintestinal localization, for example, ovarian lesions may elicit symptoms in the absence of hepatic disease owing to direct secretion into systemic circulation.(ABSTRACT TRUNCATED AT 250 WORDS)