Effects of vasoactive intestinal peptide and its homologues on the substance P-mediated secretion of fluid and protein from the rat submandibular gland.

1. Vasoactive intestinal peptide (VIP) and secretin elicited slight secretion of saliva but peptide histidine isoleucine (PHI) and gastric inhibitory peptide (GIP) failed to elicit secretion of saliva from rat submandibular glands. 2. Substance P (SP)-mediated secretion of fluid and protein were enhanced by VIP and secretin but not by PHI or GIP. 3. These results suggest that the effects of VIP, PHI, secretin and GIP on the secretion of fluid from rat submandibular glands and the synergistic effects of VIP and its homologues on the SP-mediated secretion of fluid and protein do not correspond to the extent of the structural homology of each analogue to VIP.