Regulation of the human histamine H-1 receptor stably expressed in Chinese hamster ovary cells

1 The human H-1 receptor gene expressed in Chinese hamster ovary cells (CHOhumH(1)) encodes a classical histamine H-1 receptor with a pharmacology similar to that of the H-1 receptor found in guineapig cerebellum and the endogenously expressed human H-1 receptor in 1321N1 astrocytoma cells as determined by [H-3]-mepyramine binding studies. 2 In CHOhumH(1) cells, histamine induced a concentration-dependent rise in inositol phosphates (EC(50) 2.23 +/- 0.97 mu M) and a rapid increase of [Ca2+](i), followed by a sustained increase of [Ca2+](i) upon addition of 100 mu M histamine. 3 Short-term exposure of CHOhumH(1) cells to histamine (100 mu M) resulted in a decrease of subsequent histamine-induced Ca2+ responses. The histamine-induced desensitization appeared to be heterologous as the ATP-induced Ca2+ response was also found to be affected. 4 The process of heterologous histamine-induced desensitization of the Ca2+ response in CHOhumH(1) cells can be ascribed to an alteration at the level of the intracellular Ca2+ pool, as the Ca2+ response of caffeine (10 mM), which releases Ca2+ from intracellular Ca2+ stores was also attenuated upon short-term histamine exposure. 5 In CHOhumH(1) cells the PKC activator, PMA, was found to inhibit the histamine (100 mu M)-induced Ca2+ C response concentration-dependently (IC50 0.2 +/- 0.03 mu M) as well as the ATP (100 mu M)-induced Ca2+ response. However, this inhibition was only partial and less effective than histamine-pretreatment. Moreover, in CHOhumH(1) cells PKC downregulation induced by long-term exposure to PMA (1 mu M) did not affect the histamine-induced desensitization nor did pretreatment with the specific PKC inhibitor Ro-31-8220 (10 mu M) indicating that in CHOhumH(1) cells PKC is probably not involved in the heterologous desensitization. 6 Long-term treatment of CHOhumH(1) cells with histamine or other H-1 agonists resulted in a time- and concentration-dependent decrease in the number of Hi receptor binding sires (maximal reduction: 47 +/- 5%). 7 Long-term exposure of CHOhumH(1) cells to ATP or PMA did not affect H-1 receptor density. 8 Both histamine (100 mu M)- and ATP (100 mu M)-induced Ca2+ responses were affected upon long-term exposure of cells to histamine (100 mu M), which might be explained by an alteration at a level distant from the receptor. 9 These results show that in CHOhumH(1) cells the human histamine Hi receptor is susceptible to shortterm and long-term receptor regulation in which PKC does not seem to play a role. The CHOhumH, cells therefore provide an excellent model system for studying the mechanism(s) of PKC-independent H-1 receptor regulation.