Cardiac Resynchronization Therapy Augments Myofilament Ca2+ Responsiveness in Failing Myocardium via Altered Phosphorylation

Cardiac resynchronization therapy (CRT) has become one of the major clinical therapies that improve long term survival in heart failure (HF) patients. The beneficial effects of CRT have been suggested, including upregulation of sarcoplamic reticulum ATPase, β1-adrenergic receptor expression, and improved chamber mechanoenergetics. Here, we reveal that CRT also acts as a calcium sensitizer and restores maximum force when applied in a dog model of dyssynchronous HF (DHF, 6 wks rapid atrial pacing with LBBB). Trabeculae (in mm: 0.35±0.02 wide, 0.22±0.01 thick, 1.74±0.13 long, n=27) from the ventricles of control, DHF, always synchronous HF (HF-synch, atrial rapid pacing for 6 wks), and CRT (BiV rapid pacing for latter 3 wks) were dissected, mounted between a force transducer and a motor arm, and chemically skinned in 1% Triton. Force-pCa relations were obtained with varied [Ca2+]. The Table compares maximal Ca2+- activated force (Fmax), the [Ca2+] for 50% of Fmax (Ca50), and the Hill coefficient in these groups. In parallel, myofibrillar ATPase activity as a function of Ca2+ activation were studied in these four groups (n=4, and 3 runs in each dog sample). DHF displayed a substantial downward shift with depression of minimum and maximal Ca2+-activated myofibrillar ATPase compared with control or CRT, and HF-synch myofibrils also yielded a response quite similar to that of DHF. Finally, 2D-DiGE and mass spectrometry of the myofilament proteins showed increased phosphorylation of troponin I, myosin light chain 2, and - tropomyosin (at a novel site, T237), while troponin T had less phosphorylation. These results demonstrate that (1) myofilament Ca2+ responsiveness is markedly depressed in HF and this myofilament abnormality occurs in HF per se (i.e. regardless of synchrony), (2) CRT significantly improves myofilament Ca2+ responsiveness, and (3) post-translational modification of myofil-ament proteins may underlie the beneficial myofilament effects of CRT