Type 2 phosphodiesterase regulates cardiac pacemaker activity

Elevated heart rate (HR) and lower HR variability (HRV) predict cardiovascular morbi-mortality. HR is mainly controlled by the autonomic nervous system (ANS). During sympathetic stimulation, the activation of β-adrenergic receptors increases cAMP levels leading to positive chronotropic effect. In the heart, cAMP can be hydrolyzed by 6 different phosphodiesterases (PDEs). One of them is PDE2 which has recently been reported to be beneficial for the failing heart. However, whether this is due to a direct control sinoatrial node (SAN) function or an indirect effect via the ANS is unknown. To explore the role of PDE2 in regulating HR. The hydrolytic cAMP activity was measured with radioenzymatic assay. The in vitro pacemaker activity was assessed by measuring spontaneous Ca2+ transients in Fluo4-loaded-SAN tissue using confocal microscopy. ECG telemetry was recorded in WT and cardiac PDE2-overexpressing (PDE2TG) conscious mice, in basal conditions, after ANS inhibition with atropine (atro, 1 mg/kg, ip) + propranolol (propra, 2 mg/kg, ip), or after β-adrenergic stimulation with isoprenaline (Iso, 1.5 mg/kg, ip). If current was measured by patch-clamp (whole-cell). PDE2 is expressed in WT SAN and overexpressed by ∼4-fold in the PDE2TG SAN. Total cAMP-hydrolyzing activity is increased ∼3-fold in PDE2TG SAN. PDE2TG mice display lower basal HR, during day and night. This phenotype was maintained after atro + prop and after Iso. Basal beating rate of PDE2TG SAN tissue was lower. While no difference in Ca2+ transient amplitude time to peak and duration at half maximum were unchanged in PDE2TG SAN cells, the time constant of decay was increased in basal conditions. Ca2+ sparks analysis between WT and PDE2TG SAN did not show any modification on ryanodine receptor activity. In addition, If current density was decreased in PDE2TG SAN cells. PDE2 regulates HR in vivo by a mechanism independent from ANS which takes place at the level of SAN and involves the voltage clock.