PDE4 regulates cardiac pacemaker function

Abstract

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 β-stimulation in atrial and ventricular cells. PDE4 may also be important for automaticity. 3 genes encode for PDE4: pde4a,4b,4d. Their respective contribution to the regulation of pacemaker activity remains unknown. 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-201724. The in vitro pacemaker activity was assessed by measuring spontaneous Ca2+ transients in Fluo4-loaded-SAN tissue. Images were obtained using confocal microscopy. 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 total cAMP-PDE activity in SAN ( n = 3). PDE4A, 4B and 4D isoforms were found to be expressed in mouse SAN ( n = 5 independent experiments). In 4D-, but not in 4BKO mice, Ca2+ homeostasis was altered in control conditions (ctrl) and after β-stimulation with isoprenaline (iso). Indeed, ablation of PDE4D induced decreased beating rate (ctrl: 1.00 ± 0.08s-1 vs. 1.57 ± 0.05s-1; iso: 1.71 ± 0.17s-1 vs. 2.39 ± 0.08s-1, P < 0.0001) and increased Ca2+ spark frequency (ctrl: 15.9 ± 5.2 vs. 1.9 ± 0.4sparks/s/100 μm; iso: 22.9 ± 7.1 vs. 0.6 ± 0.2sparks/s/100 μm, P < 0.0001) ( Fig. 1 ). PDE4 controls pacemaker function in mice and PDE4D ablation strongly perturbs normal SAN activity.