Asp73-dependent and -independent regulation of the affinity of ligands for human histamine H1 receptors by Na().

Abstract The affinity of ligands for G-protein-coupled receptors (GPCRs) is allosterically regulated by Na + via a highly conserved aspartate residue (Asp 2.50 ) in the second transmembrane domain of GPCRs. In the present study, we examined the Na + -mediated regulation of the affinity of ligands for G q/11 -protein-coupled human histamine H 1 receptors in Chinese hamster ovary cells. The affinities of 3 agonists and 20 antihistamines were evaluated by their displacement curves against the binding of [ 3 H]-mepyramine to membrane preparations in the presence or absence of 100 mM NaCl. The affinities of most drugs including histamine, an agonist, and d -chlorpheniramine, a first-generation antihistamine, were reduced by NaCl, with the extent of NaCl-mediated changes varying widely between drugs. In contrast, the affinities of some second-generation antihistamines such as fexofenadine were increased by NaCl. These changes were retained in intact cells. The mutation of Asp 2.50 (Asp73) to asparagine abrogated NaCl-induced reductions in affinities for histamine and d -chlorpheniramine, but not NaCl-induced increases in the affinity for fexofenadine. Quantitative structure-activity relationship (QSAR) analyses showed that these Na + -mediated changes were explained and predicted by a combination of the molecular energies and implicit solvation energies of the compounds. These results suggest that Na + diversely regulates the affinity of ligands for H 1 receptors from the extracellular sites of receptors via Asp73-dependent and -independent mechanisms in a manner that depends on the physicochemical properties of ligands. These results may contribute to a deeper understanding of the fundamental mechanisms by which the affinity of ligands for their receptors is allosterically regulated by Na + .