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. Three 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±0.4 sparks/s/100μm; iso: 22.9±7.1 vs. 0.6±0.2 sparks/s/100μm, P<0.0001) (Fig. 1).

PDE4 controls pacemaker function and PDE4D ablation strongly perturbs normal SAN activity.