title: Differences in efficacy and Na(+) sensitivity between alpha(2B) and alpha(2D) adrenergic receptors: implications for R and R* states.
abstract: The ability of Na(+) ions to modulate coupling of alpha(2B)- and alpha(2D)-adrenergic receptors to G proteins was investigated in isolated membranes from transfected PC12 and NIH 3T3 fibroblast cells. The initial rate of epinephrine-stimulated [(35)S]GTPgammaS binding was higher for alpha(2D)-receptors (the rat homolog of the alpha(2A)-receptor) in both cell types, whereas both alpha(2B)- and alpha(2D)-receptor responses were higher in PC12 cell membranes. Pertussis toxin completely blocked agonist-stimulated binding. Graded increases in Na(+) caused a progressive loss of basal GTP binding, indicative of its ability to reduce the level of the active R* state of the receptor. This inhibitory effect of Na(+) was more pronounced in PC12/alpha(2B) than PC12/alpha(2D) membranes. Epinephrine-stimulated GTP binding in PC12/alpha(2B) membranes was also more sensitive to Na(+) inhibition than in PC12/alpha(2D) membranes. In saturation [(35)S]GTPgammaS binding studies, the presence of Na(+) reduced apparent GTP affinity, and its effect was greater in PC12/ alpha(2B) membranes, consistent with a greater reduction in the active R* conformation of the receptor. The higher efficacy of epinephrine at alpha(2D) receptors and their lesser sensitivity to Na(+) are both indicative of a more stable R* state. Together these results suggest that differences in the modulatory influence of Na(+) within a family of G(i)-coupled receptors may reflect differences in the stability of the active R* state.
authors.affiliation: Department of Pharmaceutical Sciences, Northeastern University, Boston, MA 02115, USA.
authors.forename: W N
authors.initials: WN
authors.lastname: Tian
authors.forename: R C
authors.initials: RC
authors.lastname: Deth
mesh_terms: D016475:3T3 Cells; D000818:Animals; D004305:Dose-Response Relationship, Drug; D004837:Epinephrine / Q000494:pharmacology; D016244:Guanosine 5'-O-(3-Thiotriphosphate) / Q000378:metabolism; D006160:Guanosine Triphosphate / Q000378:metabolism; D051379:Mice; D016716:PC12 Cells; D051381:Rats; D018341:Receptors, Adrenergic, alpha-2 / Q000737:chemistry / Q000187:drug effects*; D012964:Sodium / Q000494:pharmacology*; D015016:Yohimbine / Q000494:pharmacology
chemical_list: D018341:Receptors, Adrenergic, alpha-2; D015016:Yohimbine; D016244:Guanosine 5'-O-(3-Thiotriphosphate); D006160:Guanosine Triphosphate; D012964:Sodium; D004837:Epinephrine