vasoactive intestinal polypeptide (VIP), a 28-amino-acid polypeptide secreted by cells throughout the intestinal tract. It stimulates the secretion of electrolytes and water by the intestinal mucosa. Some pancreatic islet-cell tumours secrete excessive amounts of VIP (a condition called Verner-Morrison syndrome, or pancreatic cholera). VIP-secreting tumours cause severe, intractable, debilitating watery diarrhea and an associated loss of large quantities of potassium. The resulting dehydration may be life-threatening.

Studies have indicated that VIP is capable of acting as a neurotransmitter, inducing a relaxation effect in some tissues. This function, though not well characterized, has been supported by research suggesting that some other gastrointestinal hormones may serve as neurotransmitters in the brain. These hormones include motilin, neuropeptide Y (which interacts with ghrelin to regulate appetite), gastrin-releasing peptide (bombesin-like peptide), glucagon, and somatostatin.