Release of tryptophan and serotonin into the portal vein of the isolated perfused rat small intestine.

To investigate the release of serotonin from intestinal enterochromaffin cells, we used an in vitro technique which allows studies excluding overlapping influences from outside the gut. The entire small intestine of rats fed a standard or tryptophan-enriched (3% of total) diet was totally isolated by ligatures with the exception of the superior mesenteric artery and portal vein that supply and drain the intestine. Simultaneously to the vascular perfusion (Krebs-Ringer bicarbonate buffer, 0.4% human albumin, 5 mM glucose, 0.6 mM glutamine) the gut lumen was infused (buffer or 0.1 N HCL). Acidification of the gut lumen resulted in an increment of venously released tryptophan and serotonin. After feeding tryptophan-enriched food the release of tryptophan was increased. However, the total amount of released serotonin after tryptophan diet did not differ as compared to that after standard diet. Addition of a monoamino-oxidase inhibitor (pargyline) to the arterial perfusate enhanced the released amount of serotonin 3-fold in the portal venous effluent (at a concentration of 1 mM but not 0.1 mM). Recovery studies done by arterial infusions of serotonin (1 microM, 10 microM) and evaluation of the amounts venously released revealed a high loss of infused serotonin (40%-70%). Our data suggest gut-born serotonin to more likely play a paracrine role than a role as a classical hormone.