I. Lack of space B-adrenergic receptors in human ventricular myocardium: II Pharmacological characterization of chick and frog myocardial B-adrenergic receptors

The alkylating agent bromoacetylalprenololmenthane (BAAM), which selectively binds beta-adrenergic receptors, was used to determine if human ventricular myocardium contains "spare" beta-receptors. Decreases in beta-receptor density were compared to changes in agonist stimulated adenylate cyclase activity and isolated muscle contractility with and without BAAM treatment. In membrane preparations, BAAM produced concentration-dependent decreases in beta-receptor density and adenylate cyclase activity. In right ventricular trabeculae from nonfailing and failing human hearts, BAAM produced modest decreases in contractile force. These results imply a lack of a detectable pool of "spare" beta-adrenergic receptors in membrane preparations when the pharmacological endpoint is adenylate cyclase activity. However, the presence of a small pool of "spare"' beta-adrenergic receptors in isolated muscle strips cannot be ruled out when the endpoint is contractile response. We used primary cultures of embryonic chick and frog myocardial cells to determine their usefulness as model systems for the eventual study of the long-term influences of catecholamines on beta-receptor pathways. Using nonlinear regression analysis, chick beta-receptors modeled for a single low affinity site based on both ICYP/betaxolol (beta1-selective) and ICYP/ICI-118551 (beta2-selective) competition curves indicating that beta-avian is neither beta1or beta2. With CGP 20712A (beta1-selective), the chick modeled for two sites, a high affinity "beta1-like" site, and a low affinity "beta2-like"' site. Also, the relative ability of agonists to displace ICYP was "beta1-like," with a rank order of potency being isoproterenol > epinephrine ? norepinephrine. For adenylate cyclase stimulation, norepinephrine was slightly less potent than epinephrine suggestive of a low affinity "beta2-like" component for norepinephrine. ICYP/antagonist competition curves in frog myocardial cells modeled for a mixed population of receptors, the subtype percentages varying from 50% to 100% beta2 depending on the antagonist used and the individual cell preparation. In agonist competition curves, the relative ability to displace ICYP was isoproterenol > epinephrine ? norepinephrine. However, in terms of their ability to stimulate adenylate cyclase activity, the order of potency was isoproterenol > epinephrine > norepinephrine, more suggestive of the presence of beta/2 receptors. Both chick and frog myocardial cells appear to be useful models for studying the effects of catecholamines on beta-receptor pathways if certain limitations are recognized.