HFWM: Regulation of cardiac pacemaker activity by PDE4 isoforms

Background Numerous epidemiological and clinical studies have revealed a positive correlation between heart rate (HR) and cardiovascular morbimortality. The autonomic nervous system is the major extracardiac determinant of HR. During sympathetic stimulation, the activation of β-adrenergic receptors (βAR) induces an increase in cAMP levels, leading to a positive chronotropic effect. Among the 5 cAMP-PDE families expressed in the heart, PDE4 is critical for controlling excitation-contraction coupling (ECC) during βAR stimulation in atrial and ventricular cells. PDE4 may also be important for automaticity. 3 genes encode for cardiac PDE4s: pde4a, pde4b and pde4d. Their respective contribution to the regulation of pacemaker activity remains ill-defined. Methods The total enzymatic PDE activity was determined in mouse sinoatrial node (SAN) tissue as the cAMP hydrolytic activity measured in the absence of PDE inhibitor and the fraction corresponding to PDE4 activity was assessed by including the PDE4 inhibitor Ro-20-1724 (10 µM). The in vitro pacemaker activity was assessed by measuring the spontaneous Ca2+ transients in Fluo4-loaded-SAN intact tissue. Images were obtained using confocal microscopy. Results Ro-20-1724 increased the beating rate of intact mouse SAN and increased PKA-phosphorylation levels of key ECC actors (ryanodine receptor, phospholamban). PDE4 enzymatic activity was found to account for 60% of the total cAMP-PDE activity in SAN. The 3 isoforms PDE4A, 4B and 4D were found to be expressed in mouse SAN. In PDE4D-, but not in PDE4B-deficient mice, Ca2+ homeostasis was altered in control conditions and after βAR stimulation. Indeed, ablation of PDE4D induced a decreased beating rate and an increased Ca2+ spark frequency in control and βAR-stimulated conditions. Conclusion Our preliminary results reveal that PDE4 controls pacemaker function in mice and that PDE4D ablation strongly perturbs normal SAN activity.