Studies with isolated liver plasma membranes indicate the presence of a guanine nucleotide- sensitive norepinephrine-binding receptor with low affinity for beta-adrenergic and alpha l-adrenergic antagonists. The receptor appears to be of the alpha 2-adrenergic subtype based on the observation that it binds the alpha 2- selective antagonist rauwolscine with high affinity (2-4 nM). Its affinity for a second alpha 2-selective antagonist yohimbine, however, is less than that observed for alpha 2-adrenergic receptors in platelet and brain preparations. Competitive binding studies with adrenergic agonists indicate that the affinity for clonidine exceeds that for both epinephrine and norepinephrine. Agonist-binding affinity is reduced in the presence of the GTP analogue, guanylyl-beta, gamma, imidodiphosphate. High affinity agonist binding and guanine nucleotide sensitivity are abolished in pertussis toxin-treated membranes indicating that the receptor signal transduction mechanism involves a guanyl nucleotide binding protein.
