MODULATION OF AGONIST BINDING TO RECOMBINANT HUMAN ALPHA(2)-ADRENOCEPTORS BY SODIUM-IONS

Agonist binding to az-adrenoceptors is modulated by a number of factor s such as Mg2+ and Na+ ions and by experimental manipulations which in terfere with receptor-G-protein-coupling such as pertussis toxin pre-t reatment or the presence of guanine nucleotides. Agonist binding assay s may therefore offer an opportunity to make inferences, albeit indire ct, about receptor states or conformations and about the molecular nat ure of the processes involved in receptor activation. We have investig ated possible differences in the effects of Na+ ions on the binding of agonists to the three human alpha(2)-adrenoceptor subtypes, alpha(2A) , alpha(2B) and alpha(2C), recombinantly expressed in S115 mouse mamma ry tumour cells. NaCl (40 mM) influenced the apparent affinity of a pa nel of alpha(2)-adrenoceptor ligands in a complex compound- and subtyp e-dependent manner. Sodium ions affected both high- and low-affinity c onformations of the receptors, as defined by co-incubation with IO IJ- M 5'-guanylylimidodiphosphate (Gpp(NH)p). The effects of NaCl and Gpp( NH)p on agonist binding were additive indicating different modes of ac tion for the two allosteric modulators. Thus, quite marked differences between closely related receptor subtypes were noted in the molecular details of agonist-receptor interactions and in the integration of al losteric modulation by Na+ ions. Possible explanations for the experim ental findings are discussed within the theoretical framework of multi state models, and a proposal is presented for a potential physiologica l role of the modulatory effect of Na+ ions, where intracellular Naf c oncentrations would direct the activating influence of receptors to di fferent G-proteins. (C) 1998 Elsevier Science B.V. All rights reserved .