Adenylate cyclase regulation in intact cultured myocardial cells

To examine the coupling of cardiac cell-surface beta-adrenergic receptors to adenylate cyclase activation and contractile response, we studied this receptor-effector response system in monolayers of spontaneously contracting chick embryo ventricular cells under physiological conditions. The hydrophilic ligand 3H-CGP12177 identified uniformly high-agonist affinity beta-adrenergic receptors (isoproterenol KD = 15 +/- 9 nM). Isoproterenol-stimulated cyclic AMP (cAMP) accumulation with 50% effective concentration at (EC50) = 12.1 nM and augmented contractile response with EC50 = 6 nM under identical conditions. One micromolar isoproterenol induced receptor loss from the cell surface with t1/2 = 13.2 min; under identical conditions cAMP content declined with t1/2 = 13.5 min and contractile response with t1/2 = 20.7 min. After agonist removal cAMP response recovered with t1/2 = 15.7 min and receptors with t1/2 = 24.7 min. Sixty minutes after agonist removal there was recovery of 52% of maximal cAMP responsiveness and 82% of the initial number of receptors; receptor occupancy was associated with 78% of initial contractile response. Agonist affinity for cell-surface receptors was changed only modestly by agonist exposure. We conclude that for this system there is relatively close coupling between high-affinity receptors, adenylate cyclase stimulation, and contractile response.