A widely distributed 28-amino-acid neuropeptide that acts as a neurotransmitter or regulator of cell function in many organs and tissues, including the central and peripheral nervous systems, heart, lungs, kidneys, and immune system. It also acts as a peptide hormone, being secreted by endocrine cells of the upper part of the small intestine in response to the entry of partially digested food from the stomach. VIP, along with secretin, stimulates the pancreas to produce a thin watery secretion containing bicarbonate. This raises the pH in the intestine in preparation for secretion of pancreatic enzymes. VIP also inhibits gastric secretion. Other physiological effects of VIP include widening of blood vessels (vasodilation), increased cardiac output, widening of the airways (bronchodilation), and relaxation of smooth muscle. In nervous tissue it can function as a cotransmitter. For example, parasympathetic neurons supplying the salivary gland secrete both acetylcholine and (at high-frequency stimulation) VIP, causing vasodilation and secretion of saliva. VIP also modulates the function of immune cells, such as macrophages and helper T cells, and has an anti-inflammatory role.