P2558PDE4 regulates cardiac pacemaker function

Abstract

Abstract 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 positive chronotropic effect. Among the 5 cardiac cAMP-PDE families, 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 PDE4s: pde4a, pde4b, pde4d. Their respective contribution to the regulation of pacemaker activity remains ill-defined. Purpose Define the role of PDE4 isoforms in the regulation of cardiac pacemaker activity Methods Total 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. The in vitro pacemaker activity was assessed by measuring spontaneous Ca2+ transients in Fluo4-loaded-SAN tissue. Images were obtained using confocal microscopy. Results Ro-20-1724 increased beating rate of intact SAN and increased PKA-phosphorylation of key ECC actors (ryanodine receptor, phospholamban and contractile proteins). PDE4 activity was found to account for 60% of the total cAMP-PDE activity in SAN (n=3 independent experiments). PDE4A, PDE4B and PDE4D isoforms were found to be expressed in mouse SAN (n=5 independent experiments). In PDE4D-, but not in PDE4B-deficient mice, Ca2+ homeostasis was altered in control conditions (ctrl) and after βAR stimulation with isoprenaline (iso). Indeed, ablation of PDE4D induced decreased beating rate (ctrl: 1.00±0.08 s–1 vs 1.57±0.05 s–1; iso: 1.71±0.17 s–1 vs 2.39±0.08 s–1, p<0.0001) and increased Ca2+ spark frequency (ctrl: 15.9±5.2 sparks/s/100 μm vs 1.9±0.4 sparks/s/100 μm; iso: 22.9±7.1 sparks/s/100 μm vs 0.6±0.2 sparks/s/100 μm, p<0.0001) (Figure). Calcium Homeostasis in SAN cells Conclusion PDE4 controls pacemaker function in mice and PDE4D ablation strongly perturbs normal SAN activity. Acknowledgement/Funding ANR, Fondation Lefoulon Delalande, CORDDIM