M. Pihlavisto et al., MODULATION OF AGONIST BINDING TO RECOMBINANT HUMAN ALPHA(2)-ADRENOCEPTORS BY SODIUM-IONS, Biochimica et biophysica acta. Molecular cell research, 1448(1), 1998, pp. 135-146
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
.