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CELLULAR AND MOLECULAR

Constitutive Activity and Ligand Selectivity of Human, Guinea Pig, Rat, and Canine Histamine H₂ Receptors

Department of Pharmaceutical/Medicinal Chemistry II, Institute of Pharmacy, University of Regensburg, Regensburg, Germany (H.P., P.G., A.K., S.D., A.B.); and Department of Pharmacology and Toxicology, Institute of Pharmacy, University of Regensburg, Regensburg, Germany (R.S.)

Previous studies revealed pharmacological differences between human and guinea pig histamine H₂ receptors (H₂Rs) with respect to the interaction with guanidine-type agonists. Because H₂R species variants are structurally very similar, comparative studies are suited to relate different properties of H₂R species isoforms to few molecular determinants. Therefore, we systematically compared H₂Rs of human (h), guinea pig (gp), rat (r), and canine (c). Fusion proteins of hH₂R, gpH₂R, rH₂R, and cH₂R, respectively, and the short splice variant of G_s, G_sS, were expressed in Sf9 insect cells. In the membrane steady-state GTPase activity assay, cH₂R-G_sS but neither gpH₂R-G_sS nor rH₂R-G_sS showed the hallmarks of increased constitutive activity compared with hH₂R-G_sS, i.e., increased efficacies of partial agonists, increased potencies of agonists with the extent of potency increase being correlated with the corresponding efficacies at hH₂R-G_sS, increased inverse agonist efficacies, and decreased potencies of antagonists. Furthermore, in membranes expressing nonfused H₂Rs without or together with mammalian G_sS or H₂R-G_s fusion proteins, the highest basal and GTP-dependent increases in adenylyl cyclase activity were observed for cH₂R. An example of ligand selectivity is given by metiamide, acting as an inverse agonist at hH₂R-G_sS, gpH₂R-G_sS, and rH₂R-G_sS in the GTPase assay in contrast to being a weak partial agonist with decreased potency at cH₂R-G_sS. In conclusion, the cH₂R exhibits increased constitutive activity compared with hH₂R, gpH₂R, and rH₂R, and there is evidence for ligand-specific conformations in H₂R species isoforms.

Address correspondence to: Dr. Roland Seifert, Department of Pharmacology and Toxicology, University of Regensburg, Universitätsstraße 31, D-93053 Regensburg, Germany. E-mail: roland.seifert{at}chemie.uni-regensburg.de