The release of 5-hydroxytryptamine from rabbit small intestine in vitro.

1. A method is described by which the lumen of intestinal segments may be perfused separately and simultaneously under identical conditions of perfusion pressure and temperature, and the rate of appearance of 5-hydroxytryptamine (5-HT) into the serosal fluid measured.2. 5-HT appeared in the serosal fluid from quiescent segments of rabbit intestine at a rate of about 4.0 ng/cm intestine/hr and about 14.0 ng/cm intestine/hr from segments performing active peristaltic movements.3. For both quiescent and active segments, when the perfusion fluid contained a monoamine oxidase inhibitor, nialamide (Niamid), the rate of appearance of 5-HT was increased and if, in addition, 5-HT was present in the perfusion fluid the rate of appearance of 5-HT in the serosal fluid was even greater.4. The rate of appearance of 5-HT from the intestine of rabbits pretreated with reserpine was about 0.8 ng/cm intestine/hr and Niamid only slightly increased this rate. However, if 5-HT as well as Niamid were present the rate of appearance of 5-HT was about 6.0 ng/cm intestine/hr.5. Possible explanations for these findings are discussed.