Serotonin<unk>  tumors produce several vasoactive substances, most prominently serotonin. It is commonly thought that serotonin is the cause of the flushing, but this is only partially correct. The flushing also results from secretion of kallikrein, the enzyme that catalyzes the conversion of kininogen to lysyl-bradykinin. The latter is further converted to bradykinin, one of the most powerful vasodilators known. Other components of the<unk>  are diarrhea (probably caused by the increased serotonin, which greatly increases peristalsis, leaving less time for fluid absorption), a pellagra-like<unk>  (probably caused by diversion of large amounts of tryptophan from synthesis of the vitamin B3 niacin, which is needed for NAD production, to the synthesis of serotonin and other 5-hydroxyindoles), fibrotic lesions of the endocardium, particularly on the right side of the heart resulting in insufficiency of the tricuspid valve and, less frequently, the pulmonary valve and, uncommonly, bronchoconstriction. The pathogenesis of the cardiac lesions and the bronchoconstriction is unknown, but the former probably involves activation of serotonin 5-HT2B receptors by serotonin. When the primary tumor is in the gastrointestinal tract, as it is in the great majority of cases, the serotonin and kallikrein are inactivated in the liver; manifestations of<unk>  do not occur until there are metastases to the liver or when the cancer is accompanied by liver failure (cirrhosis).<unk>  tumors arising in the bronchi may be associated with manifestations of<unk>  without liver metastases because their biologically active products reach the systemic circulation before passing through the liver and being metabolized. In most patients, there is an increased urinary excretion of 5-HIAA (5-hydroxyindoleacetic acid), a degradation product of serotonin. The biology of these tumors differs from many other tumor types. Ongoing research on the biology of these tumors may reveal new mechanisms for tumor development.
Carcinoid syndrome
Anaphylaxis
Carcinosis
Stress ulcer
Portal hypertension
Madarosis
Amyloidosis
Alcohol intolerance