Secretory effects of intravenous serotonin on the proximal jejunum of the awake dog

The intestinal secretory effects of hyperserotoninemia were studied in conscious dogs with cannulated, exteriorized chronic proximal jejunal segments. A basal absorptive state for water and electrolytes was significantly changed to a secretory state for water, sodium ion, chloride ion, and potassium ion when serotonin was infused intravenously at 30 μg/kg per minute. This effect disappeared with the cessation of the infusion. During infusion, whole blood serotonin concentrations were similar to those in patients with the carcinoid syndrome. Diarrhea developed in all animals studied during infusion, and significant hypokalemia also occurred. These results support a primary role for serotonin in the pathogenesis of diarrhea associated with the carcinoid syndrome.