Characterization of vasoactive intestinal peptide receptors in rabbit ciliary processes.

PURPOSE: To demonstrate a potential role for vasoactive intestinal peptide (VIP) in the regulation of ciliary process function, VIP receptors on rabbit ciliary process membranes were identified and characterized in biochemical and immunochemical studies.

METHODS

Membranes were isolated from rabbit ciliary processes, and VIP receptors were characterized by competition binding, affinity cross-linking, and N-glycanase digestion. A site-specific polyclonal antibody directed against the NH2-terminal end of the deduced sequence of the recently cloned rat VIP receptor was generated and used to identify the VIP receptor by immunoblot analysis.

RESULTS

Membranes isolated from rabbit ciliary processes exhibited a high-affinity VIP binding site (KD approximately 1 nM). Secretin and glucagon, which possess considerable primary sequence homology with VIP, were ineffective in inhibiting 125I-VIP binding to ciliary process membranes. In conjunction with the chemical cross-linking agent disuccinimidyl suberate, 125I-VIP specifically labeled a 63-kd protein in membranes from ciliary processes. This apparent size was confirmed by immunoblot analysis of ciliary body membranes using a site-specific polyclonal antibody that recognizes residues 92 to 104 of the rat VIP receptor. Digestion of the affinity-labeled receptor with N-glycanase generated an N-linked oligosaccharide free core protein of -50 kd.

CONCLUSIONS

These findings demonstrate the presence of specific VIP receptors in rabbit ciliary processes. The differences in ligand specificity and structure of the ciliary process VIP receptor, compared to VIP receptors on peripheral tissues, suggest either a specific role(s) for VIP that may be unique to the anterior segment or the existence of VIP receptor isoforms.