RyR2(R4496C) Expression Induces Sinoatrial Node Dysfunction

Catecholaminergic Polymorphic Ventricular Tachycardia (CPVT) is an arrhythmogenic disease characterized by stress-triggered syncope and sudden death. While ventricular tachycardia appears only during stress conditions, CPVT patients manifest basal sino-atrial node (SAN) dysfunction, but the underlying mechanism remains unknown. We investigated Ca2+ handling on SAN from transgenic mice expressing the CPVT ryanodine receptor R4496C mutation (RyR2R4496C) and on their wild type (WT) littermates. A 2-fold increase of Ca2+ spark frequency in RyRR4496C SAN cells was recorded in basal conditions during the diastolic periods. beta-adrenergic stimulation with 20 nM isoproterenol, multiplied by ∼10-fold the occurrence of Ca2+ sparks and Ca2+ waves. Spontaneous [Ca2+]i transients recorded by confocal microscope in isolated SAN from RyR2R4496C mice presented a slower beating rate than WT SAN and an impaired positive chronotropic response to beta-adrenergic stimulation. Moreover, isoproterenol induced the appearance of [Ca2+]i transients and action potentials pauses in 75% of RyR2R4496C SAN cells. Caffeine experiments showed that the sarcoplasmic reticulum (SR) Ca2+ load was significantly reduced in the RyR2R4496C SAN cells. In vivo telemetric recordings of electrocardiograms (ECG) identified atrial and junctional escape beats overcoming sinus pauses in RyR2R4496C mice following isoproterenol injection consistent with the pauses recorded in isolated SAN. Similar ECGs were observed in CPVT patients during exercise testing, validating the animal model. We conclude that the increased activity of the RyR2R4496C SAN cells during diastolic periods unloads the SR with Ca2+, participating in SAN dysfunction in CPVT.