Modulation of bovine aortic alpha-2 receptors by Na+, 5'-guanylylimidodiphosphate, amiloride and ethylisopropylamiloride: evidence for receptor G-protein precoupling.

The influence of Na+ and 5'-Guanylylimidodiphosphate [Gpp(NH)p] on [3H]rauwolscine binding to alpha-2 adrenergic receptors was studied in plasma membranes prepared from bovine aorta. Both Na+ and Gpp(NH)p increased [3H]rauwolscine affinity while maximal binding capacity (Bmax) was significantly increased only with Gpp(NH)p. The increase in affinity was solely due to an increase in the association rate, while dissociation rate was not altered. In contrast, Na+ and Gpp(NH)p each lowered the affinity of the agonist epinephrine for both high- and low-affinity binding sites. The effects of Na+ and Gpp(NH)p on agonist binding were additive, such that only in their combined presence was a homogeneous class of low-affinity sites observed. This indicates that alpha-2 receptor-guanine nucleotide-binding protein (G-protein) interactions are modulated by both Na+ and Gpp(NH)p but via different mechanisms. Amiloride (100 or 300 microM) and ethylisopropylamiloride (10 microM) produced dose-dependent reductions in [3H]rauwolscine affinity, and, in the case of amiloride, also reduced Bmax. Competition at the [3H]rauwolscine binding site as well as noncompetitive, allosteric effects were present. The presence of Na+ augmented the ability of amiloride to reduce Bmax, indicating a shared locus of action. These findings illustrate that vascular alpha-2 receptors can interact with G-proteins even in the absence of agonists (i.e., receptor/G-protein precoupling) and that Na+ ion concentration regulates this interaction. Amiloride can occupy a Na(+)-shared binding site, causing an allosterically induced loss of receptor binding, suggesting that ligand binding and G-protein binding depend upon common receptor features.