Vasoactive intestinal polypeptide (VIP) and pituitary adenylate cyclase-activating polypeptide (PACAP) are members of a superfamily of structurally related peptide hormones that include glucagon, glucagon-like peptides, secretin and growth hormone-releasing hormone (Harmar

adenylate cyclase-activating polypeptide (PACAP) are members of a superfamily of structurally related peptide hormones that include glucagon, glucagon-like peptides, secretin and growth hormone-releasing hormone (Harmar et al., 1998). Moreover, VIP and PACAP exert tissue-specific effects by interacting with three distinct receptors that belong to a seven trans-membrane spanning domain G-proteincoupled receptor superfamily (Harmar et al., 1998). Two of these receptors, VPAC1 and VPAC2, bind VIP and PACAP with equal affinity, whereas a third receptor, PAC1, preferentially binds PACAP. Since their discoveries (Said and Mutt, 1972; Miyata et al., 1989), VIP and PACAP have been implicated as neurotransmitters in various physiological processes, including cardiovascular function, respiration, digestion and glandular secretions in different vertebrate species (reviewed by Nilsson and Holmgren, 1994; Harmar et al., 1998). Catecholamines that are secreted into the circulation are stored in separate populations of adrenalineand 1917 The Journal of Experimental Biology 206, 1917-1927 © 2003 The Company of Biologists Ltd doi:10.1242/jeb.00350