Paradoxical Effect of Increased Diastolic Ca2+ Release and Decreased Sinoatrial Node Activity in a Mouse Model of Catecholaminergic Polymorphic Ventricular Tachycardia.

BACKGROUND:
METHODS AND RESULTS:
CONCLUSIONS:
BACKGROUND:
Abstract
Catecholaminergic Polymorphic Ventricular Tachycardia (CPVT) is
characterized by stress-triggered syncope and sudden death. CPVT patients manifest sino-atrial
node (SAN) dysfunction, the mechanisms of which remain unexplored.
We investigated SAN [Ca(2+)]i handling in mice carrying the
CPVT-linked mutation of ryanodine receptor (RyR2(R4496C)) and on their wild-type (WT)
littermates. In vivo telemetric recordings showed impaired SAN automaticity in RyR2(R4496C)
mice following Isoproterenol (ISO) injection, analogous to what was observed in CPVT patients
after exercise. Pacemaker activity was explored by measuring spontaneous [Ca(2+)]i transients
in SAN cells within the intact SAN by confocal microscopy. RyR2(R4496C) SAN presented
significantly slower pacemaker activity and impaired chronotropic response under β-adrenergic
stimulation, accompanied by the appearance of pauses (in spontaneous [Ca(2+)]i transients and
action potentials) in 75% of the cases. Ca(2+) spark frequency was increased by 2-fold in
RyR2(R4496C) SAN. Whole-cell patch-clamp experiments performed on isolated
RyR2(R4496C) SAN cells showed that L-type Ca(2+) current (I(Ca,L)) density was reduced by
~50%, an effect blunted with internal Ca(2+) buffering. ISO dramatically increased the frequency
of Ca(2+) sparks and waves by ~5 and ~10-fold, respectively. Interestingly, the sarcoplasmic
reticulum (SR) Ca(2+) content was significantly reduced in RyR2(R4496C) SAN cells in the
presence of ISO, which may contribute to stopping the "Ca(2+)-clock" rhythm generation,
originating SAN pauses.
The increased activity of RyR2(R4496C) in SAN leads to an unanticipated
decrease on SAN automaticity by Ca(2+)-dependent decrease of I(Ca,L) and SR Ca(2+)
depletion during diastole, identifying subcellular pathophysiologic alterations contributing to the
SAN dysfunction in CPVT patients.