DESENSITIZATION OF NEUROMEDIN-B RECEPTORS (NMB-R) ON NATIVE AND NMB-R-TRANSFECTED CELLS INVOLVES DOWN-REGULATION AND INTERNALIZATION

The receptor for neuromedin B (NMB-R), a mammalian bombesin-related pe ptide, is widely distributed in the central nervous system and gastroi ntestinal tract. While it is known that this receptor is coupled to ph ospholipase C, like many other phospholipase C-activating receptors, l ittle is known about regulation of the NMB-R subsequent to agonist sti mulation. We studied both native NMB-R on C-6 rat glioblastoma cells a nd wild type NMB-R cloned from rat esophageal muscle which was stably transfected into Balb/3T3 fibroblasts. Both cell types rapidly increas ed [H-3]inositol phosphates and [Ca2+](i) in response to 1 mu M NMB, w hereas preincubation with 3 nM NMB for 3 h markedly attenuated the abi lity of 1 mu M NMB, but not 1 mu M endothelin-1, to alter either cell type's biological activity. Prolonged exposure to 3 nM NMB caused a ra pid decrease in the number of NMB-R, with the maximal receptor down-re gulation seen at 24 h due to NMB-R internalization. After maximal down regulation, removal of agonist resulted in a rapid restoration of NMB- R to the cell surface of both cell types. NMB-R recovery at 6 h was bl ocked by monensin, an inhibitor of receptor recycling, but was not aff ected by cycloheximide, a protein synthesis inhibitor. Resensitization to agonist paralleled the recovery of MMB-R in both cell types, and r esensitization likewise was blocked by monensin. Our data demonstrate that the NMB-R undergoes rapid homologous desensitization consequent t o agonist stimulation, which is mediated by receptor down-regulation a nd which, in turn, is regulated by internalization. During resensitiza tion, NMB-R reappearance on the cell surface membrane is independent o f protein synthesis and is due to a recycling from an intracellular si te.