Serotonin and substance P circulate in high concentrations in patients with the carcinoid syndrome. These studies were performed to evaluate the effects of intravenous infusions of serotonin and substance P to reproduce carcinoid levels of these agents on central hemodynamics, regional blood flow (using the radioactive microsphere technique), and endogenous hormone release. Serotonin did not affect mean arterial pressure but it significantly increased cardiac output, decreased systemic vascular resistance, and redistributed regional blood flow, increasing blood flow to the heart, adrenals, fundus, and antrum. Substance P significantly decreased mean arterial pressure and systemic vascular resistance, increased cardiac output, and increased blood flow to adrenal, fundus, antrum, liver, and all muscular layers of the stomach and small bowel. Neither serotonin nor substance P affected skin blood flow, nor altered circulating levels of glucose, insulin, or gastrin. Although both of these agents seem to participate in the pathogenesis of the carcinoid syndrome, our studies suggest that it is not possible to ascribe all the hemodynamic abnormalities to either.
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