Amplification of cyclic AMP generation reveals agonistic effects of certain beta-adrenergic antagonists

Some beta-adrenergic receptor (beta AR) antagonists, in addition to blocking receptor-mediated responses, possess agonistic properties or intrinsic sympathomimetic activity (ISA). In this study we describe several techniques for amplification of cAMP levels as a measure of agonistic activity, and we apply these techniques to the study of beta AR antagonists with ISA. We show that 1) a variety of beta AR antagonists with ISA, including alprenolol and cyanopindolol, enhance cyclic AMP accumulation in S49 lymphoma cells if cells are also incubated with the diterpene forskolin; 2) beta AR blockers with ISA stimulate cAMP accumulation in the presence of a water-soluble analog of forskolin but not in the presence of 9,11-dideoxyforskolin (which does not activate adenylyl cyclase); 3) the potentiation by forskolin is not unique to S49 cells but is also observed in BC3H1 smooth muscle-derived cells; 4) stimulation of cAMP accumulation by beta-blockers with ISA occurs in S49 cells in three additional settings that do not involve the use of forskolin, after pretreatment with pertussis toxin to inactivate the inhibitory guanine nucleotide binding protein, after pretreatment with [D-Trp8]-somatostatin to sensitize adenylyl cyclase, and using a radioimmunoassay to quantitate levels of cellular cAMP. We conclude that beta AR antagonists with ISA can weakly stimulate intracellular cAMP accumulation, but this stimulation is not easily detected. Elevation of cAMP levels may account for the agonistic effects of these drugs or, at least provides a measure of stimulatory guanine nucleotide-binding protein activation by these compounds.