Absence of arrhythmogenic effects in cAMP subcellular compartments supports PDE8 as a target in atrial fibrillation

Background Targeting Phosphodiesterase 8B (PDE8B) has been proposed as a strategy to restore L-type Ca 2+ current (I Ca,L ) in patients with atrial fibrillation (AF). However, the effects of total PDE8 inhibition on other Ca 2+ handling or 3′,5′-cyclic adenosine monophosphate (cAMP) nanodomains have not been assessed and thus the potential proarrhythmic consequences are still undetermined. Objective The aim of this study was to assess whether PDE8 contributes to the regulation of cAMP signaling in other Ca 2+ handling-related compartments and to sarcoplasmic reticulum Ca 2+ released arrhythmogenic events. Results Fluorescence resonance energy transfer (FRET) live-cell imaging, confocal Ca 2+ imaging and co-immunoprecipitation (Co-IP) were performed in atrial myocytes and tissues allocated across experimental assays from a total cumulative cohort of 141 patients with or without AF (sinus rhythm, Ctl). cAMP levels were not significantly increased upon non-isoform-selective total PDE8 inhibition in the cytosol, in ryanodine receptor 2 (RyR2) or in the sarcomere nanodomains, troponin I (TpnI) and myosin binding protein C (MyBP-C). Consistently, no significant changes in the frequency of arrhythmogenic Ca 2+ sparks or Ca 2+ waves after total PDE8 inhibitor application were detected. Conclusion These results suggest that PDE8 inhibition promotes a compartment-specific regulation of cAMP signaling that exclusively affects the L-type Ca 2+ channel in human atrial myocytes. Therefore, PDE8 inhibition can be used to restore the I Ca,L in patients with AF, without promoting spontaneous arrythmogenic Ca 2+ releases in isolated human atrial myocytes.