Neurotensin binding sites in porcine jejunum: biochemical characterization and intramural localization.

Neurotensin is present in high concentrations in the mammalian gut, especially in enteroendocrine cells of the mucosa. Exogenous neurotensin has been shown to alter ion transport by the mucosa and contractile activity of intestinal smooth muscle. The purpose of this study was to determine the distribution of neurotensin binding sites within the intestinal wall. Initially, biochemical characteristics of [125I]neurotensin binding sites were determined within two preparations of the distal porcine jejunum: (1) the mucosa and submucosa, and (2) the circular and longitudinal muscle with their intramural plexuses. Ligand binding data for the preparation including the mucosa and submucosa indicated that [125I]neurotensin bound specifically to two sites having apparent equilibrium dissociation constants of approximately 0.046 and 0.37 nM. A binding site with a dissociation constant of approximately 0.38 nM was confirmed for the preparation of muscle and associated intramural plexuses. Xenopsin and neurotensin were equipotent to neurotensin in competing for these binding sites; neuromedin N was approximately 40 times less potent in the preparation of mucosa and submucosa. Receptor autoradiography was used to determine the distribution of [125I]neurotensin binding sites within the wall of the jejunum. Autoradiograms of [125I]neurotensin bound to cross sections of the proximal and distal jejunum showed that the highest densities of silver grains were associated with the internal submucosal ganglia, external submucosal plexus and myenteric ganglia. A moderate density of silver grains was associated with the circular muscle. The localization of neurotensin binding sites to submucosal ganglia is consistent with observations that neurotensin effects on active anion secretion by the mucosa are blocked by tetrodotoxin. Immunohistochemical localization of neurotensin in the porcine jejunum demonstrated a limited population of neurotensin immunoreactive cells within the mucosal epithelium. It is possible that neurotensin released from these cells in the mucosa as well as neurotensin-related peptides released from enteric neurons may be the endogenous ligands for the binding sites visualized in this study.