High affinity vasoactive intestinal peptide receptors on fetal human nonpigniented ciliary epithelial cells

The effect of vasoactive intestinal peptide (VIP) on stimulation of adenylyl cyclase in fetal human nonpigmented ciliary epithelial (NPE) and pigmented ciliary epithelial (CPE) cells was studied. 1 μM VIP elicited a 5-10 fold increase in intracellular cAMP in NPE cells from three fetal donors, but caused little or no response in CPE from two fetal donors and other ocular cell types employed as controls. Appearance of cAMP in the extracellular medium was stimulated in NPE but not in CPE in response to VIP. Both NPE and CPE gave similar cAMP responses (8-13 fold) to the β-adrenergic agonist, isoproterenol. Binding studies of [125I]VIP to intact NPE and CPE revealed that VIP bound to NPE cells at a high affinity site (KD =. 33 nM and a low affinity site (KD = 16 nM), whereas VIP bound to CPE cells only at the low affinity site (KD =18 nM). In NPE cells, VIP stimulated cAMP formation with an EC50 of 0.6-1 nM, similar to the high affinity binding site KD, with maximal stimulation at 10 nM. Four peptides with various degrees of sequence homology to VIP were also studied. Of these, PHM and PHI stimulated cAMP with EC50s of 50 and 300 nM, respectively, while secretin and glucagon stimulated only at concentrations above 0.1 μM. These results suggest that in fetal human ciliary epithelium, as in rabbit ciliary epithelium (Mittag et al., J Pharm Exp Ther 241: 230, [1987]), VIP stimulation of adenylyl cyclase is a characteristic of NPE but not CPE cells.