β1- and β2-adrenergic receptor mediated spontaneous contractions and ryanodine receptor channel opening in human failing heart.

In heart failure, β-adrenergic receptor (β-AR)-Gsα-cAMP-protein kinase A (PKA) signalling predisposes to delayed-afterdepolarisation-mediated arrhythmias. Phosphodiesterases (PDEs) hydrolyse cAMP, and PDE3 restrains β1-AR, β2-AR-mediated inotropic responses in failing hearts from β-blocker-treated patients. Whether these receptors provoke arrhythmias through ryanodine receptor (RyR2) dysregulation, and how PDE activity and prior β-blockade treatment modify this, is undefined. We sought to define how β1-AR and β2-AR activation provokes arrhythmias through RyR2 mechanisms in explanted human failing hearts, and how PDE activity and prior β-blocker therapy modulate this response. A diastolic model of ventricular arrhythmia using human right ventricular trabeculae with pacing stopped was used. Trabeculae were exposed to (-)-noradrenaline/(-)-adrenaline to activate β1-AR/β2-AR respectively with or without PDE inhibitors and monitored for spontaneous contractions. In parallel, sarcoplasmic reticulum vesicles from trabeculae were used to assess RyR2 channel function, phosphorylation and oxidation. Activation of β1-AR or β2-AR increased the frequency of spontaneous contractions, concomitant increases in diastolic RyR2 channel opening, phosphorylation and oxidation. Trabeculae from carvedilol-treated patients showed fewer β2-AR-mediated spontaneous contractions than those treated with β1-AR blockers. The PDE3 inhibitor cilostamide augmented β2-AR-mediated spontaneous contractions, RyR2 channel opening and Ser2808 phosphorylation. In human failing heart, β1-AR or β2-AR activation increased arrhythmic contractions associated with greater diastolic RyR2 channel opening, phosphorylation and oxidation. PDE3 attenuated β2-AR-mediated arrhythmic contractions, RyR2 channel opening and Ser2808 hyperphosphorylation. These findings suggest that combining β1- and β2-AR blockade with strategies that reduce RyR2 Ca2+ leak and preserve RyR2-associated PDE3 activity may improve arrhythmia control in heart failure.