Presence of melatonin in the human hepatobiliary-gastrointestinal tract

A variety of speculations about the possible origin and physiological role of the neurohormone melatonin in the gastrointestinal tract exist. However, the experimental evidence supporting any of these theories is not substant ial and are missing for humans. We studied the distribution of melatonin wh ich was measured with radioimmunoassay in the following compartments and or gans of the human hepatobiliary-gastrointestinal tract: bile (obtained by e ndoscopic retrograde cholangiopancreaticography), peripheral venous and por tal venous blood (obtained from patients undergoing liver transplantation), endoscopically derived biopsies (mainly consisting of mucosa and submucosa ) of stomach, duodenum, large intestine as well as in resected liver tissue . Melatonin concentrations in gastrointestinal mucosa were between 136 +/- 27 pg/100mg (stomach) and 243 +/- 37 pg/100mg (descending colon, each n = 5 ). Biliary melatonin concentrations (85 +/- 45 pg/ml) correlated well with plasma concentrations (55 +/- 38 pg/ml, each n = 14) and a considerable amo unt of melatonin (about 51 ng/24 hours) appears to be excreted into the gut via the bile duct. Melatonin concentrations were slightly higher in portal than in peripheral venous blood and also the liver contained higher concen trations of melatonin than the blood. In conclusion the presence and distri bution of melatonin in human gut, bile, liver and portal blood and the vari ous reports on modulatory actions of melatonin on gut and liver functions s uggest that melatonin may act as a mediator of inter-organ communication be tween gut and liver. (C) 2001 Elsevier Science Inc. All rights reserved.