Quantitative studies of vasoactive intestinal peptide (VIP) binding sites and VIP-induced adenosine 3′:5′-monophosphate production in epithelial cells from duodenum, jejunum, ileum, coecum, colon and rectum in the rat

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Abstract
The binding of vasoactive intestinal peptide (VIP) and its effect on cAMP production were measured in isolated rat intestinal epithelial cells from duodenum, jejunum, ileum, cecum, colon and rectum. In every segment tested, VIP binding and VIP-induced cAMP production exhibited a similar pattern with 2 populations of VIP binding sites and similar magnitude of cAMP production. Continuous displacement of the binding of 125I-labeled VIP and stimulation of cAMP production were observed in the range of 10-10-10-7 M VIP concentrations. Fifty percent inhibition of the initial binding of tracer was obtained with 2.2-3.2 nM VIP. The Km of stimulation of cAMP production was 1.2-2.6 nM VIP. Chicken VIP and porcine secretin showed a 4-fold higher and a 100-fold lower affinity than porcine VIP for binding sites, respectively. The same was true for the stimulation of cAMP production. The existence of receptors highly sensitive to VIP in the separate segments of the bowel corresponds to the presence of high concentration of VIP at the vicinity of epithelial cells and strongly suggests that VIP is active in regulating different intestinal functions.