Characterization of Vasoactive Intestinal Peptide Receptors in Human Colonic Epithelial Cells*

Abstract

Receptors, i. e. specific binding sites for vasoactive intestinal peptide (VIP), have been characterized in human colonic epithelial cells isolated by EDTA treatment using ¹²⁵Ilabeled porcine VIP. The binding was time andtemperature dependent. Conditions of apparent equilibrium were obtained at 15 C after 45 min of incubation in the presence of 2.1–7.4 µg cell DNA/ml; these conditions minimized the degradation of the peptide and the binding sites. Native VIP competitively inhibited the binding of [¹²⁵I]VIP in the range of 3×10⁻¹¹–10⁻⁷ M, and half-maximal inhibition was observed at 2 × 10⁻⁹ M VIP. Scatchard analysis of these data was consistent with the existence of two classes of binding sites: 7.8 × 10⁹ high affinity sites/µg DNA with a dissociation constant (K_d) of 1.4 × 10⁻⁹ M, and 12.0 × 10¹⁰ low affinity sites/µg DNA with a K_d of 46 × 10⁻⁹ M. Among the natural hormones structurally related to VIP,gastric inhibitory polypeptide (GIP) andglucagon had no effect on the binding of labeled porcine VIP. Porcine secretin inhibited [¹²⁵I]VIP binding, but at doses 1000 times higher than those of porcine VIP. Studies of the coupling between the binding of VIP and the stimulation of cAMP formation indicated a nonlinear relationship between the two processes, with full activation of the cAMP-producing system with occupancy of only a limited number of the binding sites. The presence of binding sites with high affinity for VIPcoupled with the cAMP production in human colonic epithelial cells support the concept that this peptide may contribute to the physiological regulation of the functions of the human colonic epithelium in normal and pathological conditions.