Abstract 13417: Sinus Bradycardia in the CASQ2 R33Q/R33Q Mouse Model Of CPVT is Caused by Abnormal Calcium Clock Physiology
It has been reported that Catecholaminergic Polymorphic Ventricular Tachyccardia (CPVT) patients present slower than normal heart rate however the electrophysiological abnormalities underlying this manifestation of the disease have not been clarified. Automaticity in the sinus-atrial node (SAN) is generated by I(f) current (Voltage clock) and by rhythmic spontaneous SR Ca2+ release (SCaR) from sarcoplasmic reticulum that activates the NCX and generates the depolarizing Iti inward current (Calcium clock). We therefore investigated which “clock” is impaired in CPVT. We studied SAN cells isolated from our knock-in CPVT mice carriers of the homozygous R33Q mutation in the cardiac calsequestrin gene CASQ2. In the conscious CASQ2 R33Q/R33Q mice (n=7) we found a significantly slower heart rate vs WT mice (N=7; 597±29 vs 693±32 bpm, p<0.05). Patch clamp studies showed a reduction of automaticity in R33Q SAN cells as compared with WT (211±17 vs 291±14 bpm, p<0.05, n=8 for each group). We performed Confocal Ca2+ imaging studies and showed reduction of amplitude of the spontaneous Ca2+ transients (3.01±0.25 vs 3.45±0.16 F/F0, p<0.05, n=8 SAN cells for each group) and slower decay of spontaneous Ca2+ transients (Tau 53.0±4.8 vs 41.1±2.3 ms, p<0.05, n=8 cells for each group) in R33Q vs WT SAN cells. In permeabilized cells Ca2+ sparks in R33Q SAN cells had lower amplitude and narrower FWHM (Full width at half maximum) than WT. Exposure to 100 nM [Ca2+]i (EGTA 0.05 mM), all WT SAN cells developed rhythmic propagated Ca2+ waves (0.84±0.06 Hz, n=15), while only 31.5% R33Q SAN cells (n=19) presented rhythmic propagated Ca2+ waves (0.60±0.09 Hz, p<0.05 vs WT), the remaining R33Q cells showed unsynchronized Ca2+ waves. In conclusion: SAN dysfunction in CASQ2 R33Q/R33Q mice is associated with profound abnormalities of intracellular calcium physiology. We propose that the observed attenuation of Ca2+ sparks in R33Q SAN cells impairs the synchronization of SCaR and delays the onset of membrane firing induced by NCX current thus causing bradycardia in CASQ2 R33Q/R33Q mice.
- © 2011 by American Heart Association, Inc.