Abstract 763: Human Cardiac Beta1-Adrenergic Receptors Are Desensitized By Receptor-Mediated Activation Of Phosphatidylinositol-3 Kinase and Phosphodiesterase-4
Background: Cardiac β-adrenergic receptors (β-ARs) couple to the stimulatory G-protein (Gs), activating adenylyl cyclase to produce cAMP and increase force of contraction. Animal models suggested a role for phosphatidylinositol-3 kinase (PI3K) in regulating β-AR dependent cAMP accumulation. It is unknown, however, whether in human myocardium, β1-ARs couple to PI3K, and which downstream signaling components are involved.
Methods and Results: Isometric force of contraction was determined on isolated human atrial trabeculae of 41 patients undergoing cardiac surgery (LVEF 58±3%). For β1-AR stimulation, norepinephrine (NE, 0.001–100 μmol/L) was applied after α- and β2-AR blockade. In 21 of the 41 patients, the positive inotropic effect of NE was followed by a substantial negative inotropic effect (within the same concentration of the agonist), reducing force back to baseline values. This biphasic inotropic behaviour was repetitively observed at subsequent cumulative concentrations of NE. In contrast, in the remaining 20 (of 41) patients, a monophasic positive inotropic effect (as originally expected) was observed. The potency of NE at β1-ARs was lower in hearts with biphasic vs monophasic responses (EC50: −5.8±0.1 vs −6.7±0.2 logM; p<0.0001), indicating that the secondary negative inotropic effect was associated with desensitization of β1-ARs. Inhibiting PI3K with LY294002 (5 μM) or phosphodiesterase- (PDE-) 4 with rolipram (100 nM) had no positive inotropic effect per se, however, completely abolished the secondary negative inotropic effect and restored the potency of NE on β1-ARs (myocardium with biphasic inotropic NE-effects: Control, −5.8±0.1 logM; LY294002, −6.9±0.2 logM; rolipram, −7.0±0.2; p<0.05). Combined application of LY294002 and rolipram had no additive effect on NE potency. In myocardium with monophasic NE-responses, LY294002 and rolipram had no effects, while inhibiting PDE-3 with milrinone (1 μM) shifted the EC50 of NE further to the left (−7.5±0.1 logM; p<0.05) and had a positive inotropic effect in the absence of NE.
Conclusions: In human myocardium, norepinephrine-induced stimulation of β1-ARs activates PI3K and PDE4, which antagonizes the initial positive inotropic effect and desensitizes β1-ARs.