Melatonin concentrations in the luminal fluid, mucosa, and muscularis of the bovine and porcine gastrointestinal tract

Melatonin concentrations were measured in serum, luminal fluid, and tissues of the mucosa and muscularis of the entire bovine and porcine gastrointest inal tract (GIT). In both species, GIT levels profoundly exceeded serum lev els. In pigs, melatonin was lowest in the luminal fluid and highest in the mucosa. No difference was found in various layers of bovine GIT. Compared t o pigs, cows had higher melatonin levels in the stomach and ileum, but lowe r in the cecum and colon. There was no difference in melatonin levels betwe en anterior and posterior segments of bovine GIT, whereas pigs exhibited se veral fold higher concentration of melatonin in the posterior segment (cecu m and colon). Conversely, melatonin values in the anterior segment were sig nificantly higher in cows, but in the posterior segments porcine values wer e higher. In cows, concentrations in the mucosa correlated with levels in t he muscularis. Melatonin levels in the mucosa and muscularis were higher in the rumen and reticulum than in the omasum and abomasum. The species-speci fic levels and a distinct distribution of melatonin in the layers of the di gestive tube indicates that this indole may be involved in the modulation o f gastrointestinal function of monogastric as well as polygastric ungulates , albeit in a different capacity.