VIP is a 28-amino acid peptide structurally related to secretin. It was originally isolated from intestinal extracts and shown to be a potent vasodilator. Subsequent work demonstrated that VIP is very widely distributed in the peripheral and central nervous systems, and probably should not be considered a true GI hormone.
A huge number of biological effects have been attributed to VIP. With respect to the digestive system, VIP seems to induce smooth muscle relaxation (lower esophageal sphincter, stomach, gallbladder), stimulate secretion of water into pancreatic juice and bile, and cause inhibition of gastric acid secretion and absorption from the intestinal lumen.
Certain tumors arising from the pancreatic islets or nervous tissue (called VIPomas) secrete excessive quantities of VIP, and are associated with chronic, watery diarrhea.