Impaired β-adrenergic responsiveness accentuates dysfunctional excitation-contraction coupling in an ovine model of tachypacing-induced heart failure

Non-technical summary Heart failure is where the heart is unable to pump sufficient blood in order to meet the requirements of the body. Symptoms of heart failure often first present during exercise. During exercise the blood levels of a hormone, noradrenaline, increase and activate receptors on the muscle cells of the heart known as beta-receptors causing the heart to contract more forcefully. We show that in heart failure the response to beta-receptor stimulation is reduced and this appears to be due to a failure of the beta-receptor to signal correctly to downstream targets inside the cell. However, by-passing the beta-receptor and directly activating one of the downstream targets, an enzyme known as adenylyl cyclase, inside the cell restores the function of the muscle cells in failing hearts. These observations provide a number of potential targets for therapies to improve the function of the heart in patients with heart failure.Reduced inotropic responsiveness is characteristic of heart failure (HF). This study determined the cellular Ca2+ homeostatic and molecular mechanisms causing the blunted beta-adrenergic (beta-AR) response in HF. We induced HF by tachypacing in sheep; intracellular Ca2+ concentration was measured in voltage-clamped ventricular myocytes. In HF, Ca2+ transient amplitude and peak L-type Ca2+ current (ICa-L) were reduced (to 70 +/- 11% and 50 +/- 3.7% of control, respectively, P < 0.05) whereas sarcoplasmic reticulum (SR) Ca2+ content was unchanged. beta-AR stimulation with isoprenaline (ISO) increased Ca2+ transient amplitude, ICa-L and SR Ca2+ content in both cell types; however, the response of HF cells was markedly diminished (P < 0.05). Western blotting revealed an increase in protein phosphatase levels (PP1, 158 +/- 17% and PP2A, 188 +/- 34% of control, P < 0.05) and reduced phosphorylation of phospholamban in HF (Ser16, 30 +/- 10% and Thr17, 41 +/- 15% of control, P < 0.05). The beta-AR receptor kinase GRK-2 was also increased in HF (173 +/- 38% of control, P < 0.05). In HF, activation of adenylyl cyclase with forskolin rescued the Ca2+ transient, SR Ca2+ content and SR Ca2+ uptake rate to the same levels as control cells in ISO. In conclusion, the reduced responsiveness of the myocardium to beta-AR agonists in HF probably arises as a consequence of impaired phosphorylation of key intracellular proteins responsible for regulating the SR Ca2+ content and therefore failure of the systolic Ca2+ transient to increase appropriately during beta-AR stimulation.