The ability of exogenously administered epinephrine, norepinephrine, and tyramine and stimulation of sympathetic nerves to release 5-HT from the vasculature of isolated dog intestinal segments has been investigated. These stimuli were all found to produce significant release of 5-HT. Administration of an a-receptor blocking agent (tolazoline) significantly reduced the 5-HT release by norepinephrine but not by epinephrine.
T HAS BEE:N demonstrated recently (1) that isolated segments of dog small intestine rclcasc !i-hydroxytrpptamine (5HT) into physiological solution perfusing the vasculature. Stimuli which produced smooth muscle cantraction or enhanced motility of the perfused segment increased its release of 5-HT. Such stimuli as acetylcholine, angiotensin, BaCI2, increased intraluitiinal pressure, and scratching the serosal surf ace of the gut section all significantly increased fi-HT re1eas.i. It was felt t h a t stich increased 6-HT release could simply be due to mcchanical distortion of the tissues produced by the various :,timuli.
More recent work indicates that the above explanation may be oversimplified since some stimuli that relax the intestinal musculature also release 5-HT.
Methods
Adult mongrel dogs of either sex weighing 8 to 15 Kg. were anesthetized with 15 mg./Rg. of sodium thiopcntal and 250 mg./Kg. of sodium barbital administered intravenously. The small intestine was exposed and a sinall branch of the superior niese~iteric artery with its juxtaintestinal arterial fan was cannulated with polyethylene tubing. The artery was pcrfused with warmed Krebs bic:irtmnatr solution which was aerated by bubbling with LL mixture <Jf 9Scj& oxygen and 5% carbon dioxide. I'eriuision pressure, provided by use of a Sigmamotor model T-8 constant-flow peristaltic infusion pump, was maintained at 80-100 mm. Hg arid was measured from a T-tube between the pump arid the artery by a Statham pressure transducer and recorded on an Offncr Dynograph (type RS). Since flow into the artery was held constant, changes of perfusiori pressure were then proportional to changes in arterial resistance.