Anomalous behavior of CGP 12177A on beta(1)-adrenergic receptors

CGP 12177A originally was developed as a hydrophilic antagonist to detect cell surface beta(1)- and beta(2)-adrenergic receptors, and subsequently was found to be a partial agonist for the atypical or beta(3)-adrenergic receptor. Using hamster cells stably expressing either the human beta(1)-, human beta(2)- or rat beta(1)-adrenergic receptor, we found that CGP 12177A behaved as an agonist for beta(1)-adrenergic receptors. Whereas at low concentrations, CGP 12177A behaved as an antagonist and inhibited isoproterenol stimulation of adenylyl cyclase activity, at higher concentrations, it stimulated a response even in the absence of isoproterenol. The agonistic properties of CGP 12177A were positively correlated with the level of beta(1)-adrenergic receptor expression. Thus, at low receptor densities, CGP 12177A behaved as a weak, partial agonist whereas as high receptor densities, the drug was a full agonist. At similar high densities of the beta(2)-adrenergic receptor, CGP 12177A acted only as a partial agonist. Competition binding studies to membranes from cells expressing beta(1)-adrenergic receptors indicated that similar to 90% of the receptors were in a high affinity, guanine nucleotide-insensitive state for CGP 12177A whereas similar to 10% of the receptors were in a lower affinity, guanine nucleotide-sensitive state for CGP 12177A. We propose that the latter receptors are precoupled to stimulatory G proteins and recognize CGP 12177A as an agonist whereas the high affinity, uncoupled receptors recognize CGP 12177A as an antagonist.