Characterization of [H-3]CGP 12177 binding to beta-adrenergic receptors inintact eel hepatocytes

The aim of this study was to characterize [ H-3]CGP 12177 (CGP) binding to beta -adrenergic receptors in isolated hepatocytes of the European eel (Ang uilla anguilla), in which the involvement of cAMP in epinephrine-induced gl ucose release has been previously observed. Specific binding of CGP was sat urable, reversible, acid linear as a function of cell number. Analysis of b inding data suggested a single class of binding sites, with a K-d of 1.31 n M and a number of approximately 7000 beta -adrenergic receptors per cell. T he potency order of specific inhibition of [H-3]CGP binding was CGP > propr anolol greater than or equal to alprenolol >> butoxamine greater than or eq ual to atenolol, while phentolamine and prazosin failed to significantly di splace the tracer at concentrations up to 100 muM. The binding kinetics of CGP were closely related to its biological effect. In fact, the drug dose-d ependently counteracted the enhancement of intracellular cAMP levels induce d by epinephrine in isolated hepatocytes with a K-d of 1.06 nM. Moreover, i t antagonized the hormone-induced stimulation of adenylyl cyclase activity in hepatic membranes as well as of glucose release from cells. These data c learly show that beta -adrenergic receptors are coupled to the adenylyl cyc lase/cAMP transduction pathway in eel liver. (C) 2001 Academic Press.