Abstract
Long-term administration of histamine H2 receptor (H2R) antagonist (inverse agonist) induces up-regulation of the H2R, which may be relevant to rebound hypersecretion of gastric acid after withdrawing the treatment. However the underlying mechanisms remains to be elucidated. To clarify the mechanisms of antagonist-induced up-regulation of H2R, human H2R-GFP fusion protein (H2R-GFP) was generated, functionally expressed in HEK293 cells, and used for the experiments. Binding affinity of [3H]tiotidine was not significantly different between H2R-GFP and wild-type H2R expressed in HEK293 cells. Visualization of H2R-GFP revealed that antagonist induced recycling of both the agonist-induced internalized H2R and the constitutively internalized H2R, localizing in recycling endosome, within 2 hours. Persistent treatment with antagonist for at least 3 hours induced up-regulation of H2R-GFP estimated by binding assay, and also induced the increase in GFP fluorescence in plasma membrane under a microscope. During the treatment with cimetidine, an H2R antagonist, H2R mRNA was not augmented, and after its pretreatment, the inhibition of protein synthesis with cycloheximide had no effect on maintaining H2R up-regulation. These findings suggested that, upon antagonist (inverse agonist) exposure, alteration of the intracellular cycling between receptor endocytosis and recycling precedes the H2R up-regulation, probably with suppressing H2R degradation. [Jpn J Physiol 54 Suppl:S138 (2004)]
