Heritable cardiomyopathy (HCM) is the leading cause of sudden cardiac arrest (SCA) in young people, affecting 1 in 500 individuals. HCM is chiefly caused by mutations in myofibrillar proteins of the cardiac sarcomere, and cardiac myosin binding protein-C (cMyBP-C, encoded by MYBPC3) is one of the most commonly affected. cMyBP-C, an accessory protein that binds tightly to myosin, has an important role in thick filament regulation. Mice with genetic ablation of MYBPC3 exhibit cardiac hypertrophy, reduced ejection fraction, and increased relaxation times
in vivo
. Experiments with explanted hearts from these mice exhibit greater susceptibility to arrhythmias compared to WT, suggesting derangement of Ca
2+
handling. The molecular mechanisms underlying the progression of HCM are poorly understood, and are difficult to tease apart in constitutive knock out models due to potential compensatory changes that can mask important aspects of the disease phenotype. We used a tamoxifen-induced conditional MYBPC3 knock out mouse to investigate the onset and progression of the disease before development of overt hypertrophy. We recorded Ca
2+
transients in enzymatically isolated cardiomyocytes at several pacing frequencies using a META LSM 510 confocal microscope. We found that cell shortening in conditional CMs (con-CMs) was reduced by as much as 54% (e.g., 5.46% ± 0.869 in con-CMs, vs. 11.91% ± 0.423 shortening in WT-CMs). Ca
2+
release was often dissynchronous, possibly indicating remodeling at the diads. Ca
2+
transient amplitude was significantly lower in con-CMs across all pacing frequencies (reduced by at least 22%; e.g., fluorescence intensity ratios of 7.983 ± 0.537 in con-CMs vs. 10.332 ± 0.502 in WT CMs). Further, the time constant of transient decay (tau) was dramatically increased in con-CMs (mean 217.27 ± 18.052 msec for con-CMs vs. 107.44 ± 4.045 msec in WT CMs). In some cells (∼5%), this prolongation of transient decay was so profound (e.g., up to 600 milliseconds) that a second Ca
2+
release occurred before [Ca
2+
]
i
could return to baseline. These data suggest that conditional knock out of cMyBP-C produces not only contractile dysfunction, but also pro-arrhythmic alterations in Ca
2+
handling.
Calcium Handling Derangement is Associated with Conditional Cardiac Myosin Binding Protein C Knock Out
