Neuromedin B receptor in esophagus: Evidence for subtypes of bombesin receptors

To identify receptors for bombesin-related peptides in the rat esophagus, we measured binding of ¹²⁵I-Bolton-Hunter neuromedin B (¹²⁵I-BH-neuromedin B) and ¹²⁵I-[Tyr⁴]bombesin to tissue sections from the rat esophagus and compared the results with those for rat pancreas. Esophagus bound both tracers, whereas pancreas bound only ¹²⁵I-[Tyr⁴]bombesin. In each tissue binding was saturable, dependent on pH, on time, and on temperature, reversible, and specific. Autoradiography demonstrated binding of both tracers only to the muscularis mucosae of the esophagus and binding of ¹²⁵I-[Tyr⁴]bombesin diffusely over pancreatic acini. In the esophagus, the relative potencies for inhibition of binding of both tracers were as follows: neuromedin B > bombesin > GRP = neuromedin C; similar relative potencies were found for causing contraction of muscle strips from whole esophagus and from the isolated muscularis mucosae. In pancreas tissue sections and dispersed acini, the relative potencies for inhibition of binding of ¹²⁵I-[Tyr⁴]bombesin were as follows: bombesin > GRP = neuromedin C >> neuromedin B. Similar relative potencies were found for stimulation of enzyme secretion from dispersed pancreatic acini. Computer analysis in both tissues demonstrated only a single binding site. The present study demonstrates that rat esophagus muscle possesses specific receptors for bombesin-related peptides. Furthermore, this study shows that the esophageal bombesin receptors represent a previously unidentified class of bombesin receptors in that they have a higher affinity for neuromedin B than for bombesin. In contrast, the pancreatic bombesin receptors have, like all other bombesin receptors described to date, a high affinity for bombesin, but low affinity for neuromedin B.