Differences in efficacy and Na+ sensitivity between alpha(2B) and alpha(2D) adrenergic receptors - Implications for R and R* states

The ability of Na+ ions to modulate coupling of alpha(2B)- and alpha(2D)-ad renergic receptors to G proteins was investigated in isolated membranes fro m transfected PC12 and NIH 3T3 fibroblast cells. The initial rate of epinep hrine-stimulated [S-35]GTP gamma S binding was higher for alpha(2D)-recepto rs (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 m embranes. Pertussis toxin completely blocked agonist-stimulated binding, Gr aded increases in Na+ caused a progressive loss of basal GTP binding, indic ative of its ability to reduce the level of the active R* state of the rece ptor. This inhibitory effect of Na+ was more pronounced in PC12/alpha(2B) t han PC12/alpha(2D) membranes, Epinephrine-stimulated GTP binding in PC12/al pha(2B) membranes was also more sensitive to Na+ inhibition than in PC12/al pha(2D) membranes. In saturation [S-35]GTP gamma S binding studies, the pre sence of Na+ reduced apparent GTP affinity, and its effect was greater in P C12/alpha(2B) membranes, consistent with a greater reduction in the active R* conformation of the receptor. The higher efficacy of epinephrine at alph a(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 m ay reflect differences in the stability of the active R* state, Copyright ( C) 2000 S. Karger AG, Basel.