Two distinct pathways for histamine H2 receptor down-regulation. H2 Leu124 --> Ala receptor mutant provides evidence for a cAMP-independent action of H2 agonists.

Pretreatment of Chinese hamster ovary cells expressing the histamine H2 receptor (CHOrH2 cells) with histamine resulted in a time-dependent (t1/2 approximately 7 h) and dose-dependent (EC50=18 nM) H2 receptor down-regulation measured as [125I]iodoaminopotentidine binding (44+/-10% down-regulation). Pretreatment of CHOrH2 cells with cholera toxin or forskolin also led to H2 receptor down-regulation. Forskolin time-dependently (t1/2 approximately 7 h) and dose-dependently (EC50 = 0.3 microM) induced H2 receptor down-regulation. Both histamine and forskolin induced rapid down-regulation of H2 receptor mRNA levels, probably caused by mRNA destabilization. Recently, Moro et al. (Moro, O. Lameh, J., Hogger, P., and Sadée, W. (1993) J. Biol. Chem. 268, 22273-22276) showed that hydrophobic amino acids in a conserved G-protein-coupled receptor motif in the second intracellular loop are implicated in G-protein coupling. To uncouple the H2 receptor from the Gs-protein, we introduced the Leu124 --> Ala mutation in the second intracellular loop of the H2 receptor. The H2 Leu124 --> Ala mutant showed altered agonist-binding parameters, attenuated histamine-induced cAMP production, and was down-regulated by concentrations of histamine that did not give rise to cAMP production. Taken together, in CHOrH2 cells, H2 receptor down-regulation appears to be induced by two distinct pathways, a cAMP-dependent and cAMP-independent pathway.