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

Abstract

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 Gsα, GsαS, were expressed in Sf9 insect cells. In the membrane
steady-state GTPase activity assay, cH₂R-GsαS but neither
gpH₂R-GsαS nor rH₂R-GsαS showed the hallmarks of increased
constitutive activity compared with hH₂R-GsαS, 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-GsαS, increased inverse agonist efficacies, and decreased potencies of antagonists. Furthermore, in membranes expressing nonfused H₂Rs without or together with mammalian GsαS or H₂R-Gsα 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-GsαS, gpH₂R-GsαS, and rH₂R-GsαS in the GTPase assay
in contrast to being a weak partial agonist with decreased
potency at cH₂R-GsαS. 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.