An agonist specific effect of guanine nucleotides on binding to the beta adrenergic receptor

The authors have previously characterized the binding of [¹²⁵I] iodohydroxybenzylpindolol to beta adrenergic receptor sites in two clonal lines of cultured cells. They now report that whereas guanine nucleotides have no effect on the binding of this iodinated antagonist, GTP, GDP, and guanylyl imidodiphosphate all impair the ability of beta adrenergic agonists to inhibit the binding of [¹²⁵I]iodohydroxybenzylpindolol. The nucleotides have no effect on the ability of other beta adrenergic antagonists to inhibit binding. Thus, with this method, beta adrenergic agonists and antagonists can be distinguished by means of binding alone; no assessment of 'effect' need be made.