Effects of β-adrenergic blockade on papillary muscle function and the β-adrenergic receptor system in noninfarcted myocardium in compensated ischemic left ventricular dysfunction

Background. β-Adrenergic receptor blockade has been reported to improve hemodynamics and β-adrenergic receptor-adenylate cyclase function in idiopathic dilated cardiomyopathy. The purpose of this study was to determine the effects of β-adrenergic receptor blockade on the β-adrenergic receptor system and myocardial function in a model of compensated ischemic heart failure. Methods and Results. We examined the effects of propranolol treatment on the β-adrenergic receptor-adenylate cyclase system and isolated papillary muscle isometric function in noninfarcted left ventricular myocardium in rats after coronary artery ligation. In untreated rats with large myocardial infarction (MI), developed tension (DT) (3.0±0.7 versus 5.1±1.1 g/mm²), peak rate of tension rise (+dT/dt) (40.3±9.5 versus 71.2±12.0 g/mm²/sec), and peak rate of tension fall (-dT/dt) (24.4±5.0 versus 38.2±6.0 g/mm²/sec) were decreased (p<0.05). In addition, DT, +dT/dt, and -dT/dt of untreated MI rats demonstrated an impaired response to isoproterenol stimulation compared with controls. β-Adrenergic receptor density (B_max) measured by [¹²⁵I]iodocyanopindolol (ICYP) binding was decreased 23% after infarction (9.3±0.6 versus 12.0±1.8 fmol/mg protein [prot]) (p<0.05); however, the dissociation constant (K_d) for ICYP was not changed (24.1±5.7 versus 33.2±12.1 pM). Adenylate cyclase activity in the presence of 10^-2 M MgCl₂ was reduced (p<0.05) in MI rats (30.3±10.8 versus 45.9±12.5 pmol cAMP/min/mg prot). Maximal isoproterenol (52.5±7.3 versus 79.9±10.0 pmol cAMP/min/mg prot), guanyl-5′-imidodiphosphate (GppNHp) (95±8 versus 141±25 pmol cAMP/min/mg prot) and forskolin (503±76 versus 753±157 pmol cAMP/min/mg prot) stimulation of adenylate cyclase was also decreased (p<0.05). In addition, manganese-stimulated adenylate cyclase activity was depressed (p<0.05) in MI rats compared with controls (23.5±2.8 versus 52.1±9.0 pmol cAMP/min/mg prot). Chronic propranolol treatment in MI rats improved DT (4.1±0.9 versus 3.0±0.7 g/mm²) and +dT/dt (54.4±11.3 versus 40.5±9.5 g/mm²/sec) (p<0.05); however, isoproterenol-stimulated isometric function remained impaired. Propranolol treatment normalized B_max (11.9±1.7 versus 9.3±0.6 fmol/mg prot) (p<0.05), whereas adenylate cyclase activity remained depressed. Conclusions. After large MI in rats, there is impaired papillary muscle function with decreased β-adrenergic receptors and adenylate cyclase activity in the noninfarcted myocardium. Propranolol treatment improved basal isometric muscle function and β-adrenergic receptor density in rats after myocardial infarction but did not improve adenylate cyclase activity or isoproterenol-stimulated muscle function. These data suggest that there is a primary defect in adenylate cyclase function that persists despite upregulation of receptors with propranolol treatment.