VIP peptides cause vasodilation, lowers arterial blood pressure, stimulates myocardial contractility, increases glycogenolysis and relaxes the smooth muscle of trachea, stomach, and gall bladder. The vasoactive intestinal peptide (VIP) is a 28-aa long peptide corresponding to aa125-152 of the human VIP precursor after cleavage. VIP is a neuropeptide synthesized in the central nervous system, which enhances the formation of adenosine 3′-5′-cyclic monophosphate in cerebral cortex and hypothalamus. VIP also modulates the cholinergic as well as serotonergic neurotransmitter systems.

In 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. Its role in the intestine is to greatly stimulate secretion of water and electrolytes, as well as relaxation of enteric smooth muscle, dilating peripheral blood vessels, stimulating pancreatic bicarbonate secretion, and inhibiting gastrin-stimulated gastric acid secretion. These effects work together to increase motility. It also has the function of stimulating pepsinogen secretion by chief cells. VIP seems to be an important neuropeptide during inflammatory bowel diseases since the communication between mast cells and VIP in colitis, as in Crohn’s disease, is upregulated.

It is also found in the heart and has significant effects on the cardiovascular system. It causes coronary vasodilation as well as having a positive inotropic and chronotropic effect. Research is being performed to see if it may have a beneficial role in the treatment of heart failure.

VIP is also found in the brain and some autonomic nerves:

One region includes a specific area of the suprachiasmatic nuclei (SCN), the location of the ‘master circadian pacemaker’. See SCN and circadian rhythm below. VIP in the pituitary helps to regulate prolactin secretion; it stimulates prolactin release in the domestic turkey. Additionally, the growth-hormone-releasing hormone (GH-RH) is a member of the VIP family and stimulates growth hormone secretion in the anterior pituitary gland.