title: Mepyramine, a histamine H1 receptor inverse agonist, binds preferentially to a G protein-coupled form of the receptor and sequesters G protein.
abstract: Accurate characterization of the molecular mechanisms of the action of ligands is an extremely important issue for their appropriate research, pharmacological, and therapeutic uses. In view of this fact, the aim of the present work was to investigate the mechanisms involved in the actions of mepyramine at the guinea pig H(1) receptor stably expressed in Chinese hamster ovary cells. We found that mepyramine is able to decrease the basal constitutive activity of the guinea pig H(1) receptor, to bind with high affinity to a G(q/11) protein-coupled form of the receptor and to promote a G protein-coupled inactive state of the H(1) receptor that interferes with the G(q/11)-mediated signaling of the endogenously expressed ATP receptor, as predicted by the Cubic Ternary Complex Model of receptor occupancy. The effect of mepyramine on ATP-induced signaling was specifically neutralized by Galpha(11) overexpression, indicating that mepyramine is able to reduce G protein availability for other non-related receptors associated with the same signaling pathway. Finally, we found a loss of mepyramine efficacy in decreasing basal levels of intracellular calcium at high Galpha(11) expression levels, which can be theoretically explained in terms of high H(1) receptor constitutive activity. The whole of the present work sheds new light on H(1) receptor pharmacology and the mechanisms H(1) receptor inverse agonists could use to exert their observed negative efficacy.
authors.affiliation: Laboratorio de Radioisótopos, Facultad de Farmacia y Bioquímica, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Buenos Aires 1113, Argentina.
authors.forename: Carlos P
authors.initials: CP
authors.lastname: Fitzsimons
authors.forename: Federico
authors.initials: F
authors.lastname: Monczor
authors.forename: Natalia
authors.initials: N
authors.lastname: Fernández
authors.forename: Carina
authors.initials: C
authors.lastname: Shayo
authors.forename: Carlos
authors.initials: C
authors.lastname: Davio
mesh_terms: D000255:Adenosine Triphosphate / Q000378:metabolism; D000818:Animals; D015153:Blotting, Western; D016466:CHO Cells; D002118:Calcium / Q000737:chemistry / Q000378:metabolism; D002462:Cell Membrane / Q000378:metabolism; D003001:Cloning, Molecular; D006224:Cricetinae; D004305:Dose-Response Relationship, Drug; D043802:GTP-Binding Protein alpha Subunits, Gq-G11 / Q000378:metabolism; D019204:GTP-Binding Proteins / Q000378:metabolism*; D016244:Guanosine 5'-O-(3-Thiotriphosphate) / Q000378:metabolism; D006168:Guinea Pigs; D006634:Histamine H1 Antagonists / Q000494:pharmacology*; D007295:Inositol Phosphates / Q000378:metabolism; D008024:Ligands; D008956:Models, Chemical; D011485:Protein Binding; D011738:Pyrilamine / Q000494:pharmacology*; D011969:Receptors, Histamine H1 / Q000737:chemistry*; D014311:Triprolidine / Q000494:pharmacology
chemical_list: D006634:Histamine H1 Antagonists; D007295:Inositol Phosphates; D008024:Ligands; D011969:Receptors, Histamine H1; D014311:Triprolidine; D016244:Guanosine 5'-O-(3-Thiotriphosphate); D000255:Adenosine Triphosphate; D019204:GTP-Binding Proteins; D043802:GTP-Binding Protein alpha Subunits, Gq-G11; D011738:Pyrilamine; D002118:Calcium