1. The release of vasoactive intestinal polypeptide (VIP) into blood from the gastrointestinal tract was studied when eliciting autonomic nervous effects known to be mediated via non‐adrenergic, non‐cholinergic nerve fibres. All studies were performed on animals given atropine. 2. Electrical stimulation of the low threshold vagal fibres to the stomach did not significantly change gastric volume or VIP concentration in the venous effluent from the stomach. Stimulating the high threshold fibres, on the other hand, produced a gastric relaxation concomitant with a significant increase of venous plasma VIP concentrations. When eliciting a similar vagal relaxation of the stomach by distending a balloon the oesophagus a significant increase of venous plasma VIP concentration was also recorded. 3. Mechanical stimulation of the mucosa of the small bowel increased intestinal blood flow and a significant increase of venous plasma VIP concentration was observed. 4. Stimulation of the pelvic nerves to the colon produced a transient vasodilation and a significant increase of VIP in the venous effluent from the large bowel. A maintained vasodilation in the colon was induced by mechanically stimulating the rectal mucosa. This vascular response was accompanied by a significant raise of venous plasma VIP concentration. 5. The results demonstrate that all the studied nervous effects known to be mediated via non‐adrenergic, non‐cholinergic nerve fibres were accompanied by significant increases of the VIP concentration in the venous effluent. The possible physiological implications of these findings are discussed and it is proposed that VIP may be a neurotransmitter in the gastrointestinal tract.

Fahrenkrug, J., Haglund, U., Jodal, M., Lundgren, O., Olbe, L., & de Muckadell, O. B. (1978). Nervous release of vasoactive intestinal polypeptide in the gastrointestinal tract of cats: possible physiological implications. The Journal of Physiology, 284(1), 291–305. Portico.