Abstract 2634: Abnormal Calcium Handling in Casq2R33q+/+ Myocytes From Knock in Mice: Insights Into Arrhythmogenesis in Catecholaminergic Polymorphic Ventricular Tachycardia
Backgrounds: Mutations in calsequestrin 2 (CASQ2) are responsible for autosomal recessive catecholaminergic polymorphic ventricular tachycardia (CPVT). We previously demonstrated that our knock-in mouse model carrier of the CASQ2 R33Q mutation mimics the clinical phenotype of CPVT. Several mechanisms have been advocated to account for the phenotype observed in CASQ2R33Q mice ranging from defect in polymerization and calcium binding to a direct action of CASQ2R33Q on the ryanodine receptor. In the present study we studied for the first time calcium handling properties in CASQ2R33Q+/+ myocytes.
Methods and results: Ca2+ transients were recorded in fura-2-loaded myocytes during field stimulation (1–3 Hz). At 3 Hz spontaneous Diastolic Calcium Transients (DCT) occurred in 3/20 (15%) CASQ2R33Q+/+ myocytes (n=20) and in none of the WT myocytes (P<0.05); exposure to 30 nM Iso induced DCT in 54% CASQ2R33Q+/+ myocytes (n=24) and in 4% WT myocytes (n=24) (P<0.01). Rapid caffeine (10 mM) administration, showed that SR Ca2+ load was moderately decreased in CASQ2R33Q+/+ myocytes (1.68±0.04 Fratio) compared to CASQ2R33Q−/− myocytes (1.82±0.03 Fratio) (P <0.05), despite the content of CASQ2 is decreased by about 50% in CASQ2R33Q+/+ myocytes (Rizzi et al., 2008). Spontaneous Ca sparks in Fluo-4 loaded cells (spark/s/100um) occurred more frequently in CASQ2R33Q+/+ myocytes (2.39±0.51) than WT cells (0.97±0.27, P<0.05) at 1Hz. At 3Hz in the presence of 30 nM Iso, Ca sparks occurred 8.49±0.55 in CASQ2R33Q+/+ cells and 1.82±0.22 in WT cells, P<0.001.
Conclusions: Not withstanding a moderate decrease of SR Ca2+ load, CASQ2R33Q+/+ myocytes display a much higher frequency of spontaneous and Iso-induced Ca2+ sparks than WT myocytes. Thus spontaneous SR Ca2+ leak occurs in CASQ2R33Q+/+ myocytes, indicating that CASQ2R33Q can direct regulate the RyR2 channel by shifting the Ca2+ sensitivity of RyR2 (Qin et al., 2008).