Background: Cyclic guanosine monophosphate (cGMP) is a ubiquitous second messenger present in many tissues. Its signalling pathway is regulated by phosphodiesterases (PDEs), such as PDE5. While the role of PDE5 has been extensively studied in various tissues and models, its function in the human heart remains unexplored. Recent studies have reported protective effects of PDE5 inhibitors, but there have been conflicting reports of episodes of atrial fibrillation (AF) following the use of sildenafil, a PDE5 inhibitor used to treat erectile dysfunction. The aim of this study was to determine how PDE5 regulates cGMP levels in human atrial myocytes and how this regulation affects atrial cellular electrophysiology with a special focus on AF.
Materials and Methods: Human atrial myocytes were isolated from atrial tissue of patients in sinus rhythm (SR) or with AF. Cells were transduced with adenoviral vectors expressing FRET-based biosensors to monitor real-time changes in cGMP and cAMP. Voltage-clamp experiments were performed to assess L-type Ca²⁺ current, sarcoplasmic reticulum Ca²⁺ load, and spontaneous transient inward currents (ITi). Confocal Ca²⁺ imaging was employed to evaluate Ca²⁺ sparks and waves. Sildenafil (1 μM) was used to inhibit PDE5 and IBMX (100 μM) as a non-selective PDE inhibitor. Gene and protein expression were analysed in whole tissue samples via qRT-PCR and Western blot.
Results: PDE5 inhibition induced an increase in cGMP levels in both groups, however, this increase was lower in cells from AF patients. In myocytes from patients in SR, PDE5 inhibition was associated with antiarrhythmic effects, such as reduced Ca²⁺ sparks and waves, decreased ITi frequency, and lower sarcoplasmic Ca²⁺ content. In contrast, sildenafil elicited proarrhythmic responses in the cells obtained from patients with AF, enhancing spontaneous Ca²⁺ release events and ITi frequency.
Conclusion: These findings reveal that AF-associated molecular remodelling includes alterations affecting PDE5 and cGMP signalling in human atrial myocytes. The differential responses to PDE5 inhibition between SR and AF patients provide a mechanistic explanation for the conflicting clinical observations. These findings also highlight the importance of re-evaluating the safety and efficacy of PDE5 inhibitors as potential antiarrhythmic agents.
