Ca2+‐dependent down‐regulation of human histamine H1 receptors in Chinese hamster ovary cells

Gq/11 protein‐coupled human histamine H1 receptors in Chinese hamster ovary cells stimulated with histamine undergo clathrin‐dependent endocytosis followed by proteasome/lysosome‐mediated down‐regulation. In this study, we evaluated the effects of a sustained increase in intracellular Ca2+ concentrations induced by a receptor‐bypassed stimulation with ionomycin, a Ca2+ ionophore, on the endocytosis and down‐regulation of H1 receptors in Chinese hamster ovary cells. All cellular and cell‐surface H1 receptors were detected by the binding of [3H]mepyramine to intact cells sensitive to the hydrophobic and hydrophilic H1 receptor ligands, mepyramine and pirdonium, respectively. The pretreatment of cells with ionomycin markedly reduced the mepyramine‐ and pirdonium‐sensitive binding sites of [3H]mepyramine, which were completely abrogated by the deprivation of extracellular Ca2+ and partially by a ubiquitin‐activating enzyme inhibitor (UBEI‐41), but were not affected by inhibitors of calmodulin (W‐7 or calmidazolium) and protein kinase C (chelerythrine or GF109203X). These ionomycin‐induced changes were also not affected by inhibitors of receptor endocytosis via clathrin (hypertonic sucrose) and caveolae/lipid rafts (filipin or nystatin) or by inhibitors of lysosomes (E‐64, leupeptin, chloroquine, or NH4Cl), proteasomes (lactacystin or MG‐132), and a Ca2+‐dependent non‐lysosomal cysteine protease (calpain) (MDL28170). Since H1 receptors were normally detected by confocal immunofluorescence microscopy with an antibody against H1 receptors, even after the ionomycin treatment, H1 receptors appeared to exist in a form to which [3H]mepyramine was unable to bind. These results suggest that H1 receptors are apparently down‐regulated by a sustained increase in intracellular Ca2+ concentrations with no process of endocytosis and lysosomal/proteasomal degradation of receptors. We investigated effects of receptor‐bypassed increases in intracellular Ca2+ concentrations with a Ca2+ ionophore, ionomycin, on the number of human histamine H1 receptors in CHO cells. Ionomycin induced down‐regulation of H1 receptors via Ca2+‐dependent but calmodulin/protein kinase C/calpain‐independent mechanisms with no apparent endocytosis and lysosomal/proteasomal degradation of receptors in a manner different from those induced by receptor agonist.