Abstract 17270: The Effects of Cardiac Resynchronization Therapy on Myofilament Calcium Sensitivity are Global and Mediated by Protein Phosphorylation

Abstract

We have previously shown that calcium sensitivity is significantly decreased with dyssynchrony, but is improved with cardiac resynchronization therapy (CRT) using trabeculae from the right ventricle (RV). However, dyssynchrony and resynchronization cause regional changes in stress, with the RV and left ventricle (LV) experiencing differing loads. To investigate the regional effects of CRT, we used 3 canine models: dyssynchronous heart failure (HF dys , left bundle ablation, 6 wks atrial tachypacing), CRT (left bundle ablation, 3 wks atrial tachypacing, 3 wks biventricular tachypacing), and non-paced controls. Steady-state force-calcium relationships were measured in skinned myocytes isolated from the LV lateral wall. Compared to control, HF dys exhibited decreased maximal calcium-activated force (F max , Con: 35.00 ± 4.61 93 mN•mm -2 , n = 7 myocytes from 3 dogs, HF dys : 13.44 ± 1.66 mN•mm -2 , n = 7 myocytes from 3 dogs, P < 0.01), and decreased calcium sensitivity (EC 50 , Con: 1.92 ± 0.16 μM, HF dys : 2.58 ± 0.11, P < 0.01). These parameters were returned to near control values in the CRT dog (F max : 36.16 ± 2.78 mN•mm -2 , EC 50 : 2.00 ± 0.19 μM, n = 8 myocytes P < 0.05 vs HF dys , P = N.S. vs Con). To discover if these changes were mediated by altered phosphorylation state, we treated skinned RV trabeculae muscles with the phosphatase PP1 (2,500 U/ml) for 60 minutes, and measured force-calcium curves before and after treatment. None of the groups had a statistically significant change in F max after treatment ( P = N.S.). However, there was an increase in EC 50 in both Con (ΔEC 50 : 1.09 ± 0.22 μM, n = 6) and CRT (ΔEC 50 : 0.70 ± 0.23 μM, n = 5), while there was no effect on HF dys trabeculae (ΔEC 50 : -0.06 ± 0.14 μM, n = 5, P < 0.05 vs Con and CRT). This suggests that myofilament protein phosphorylation is responsible for the changes in calcium sensitivity. In summary, these results show that despite opposing regional changes in stress, the myofilament responds in a globally homogenous manner and is mediated by altered global kinase/phosphatase activity, and not by local stress response. Overall, this is encouraging data for the development of a pharmacological intervention to recreate the beneficial aspects of CRT, since regional targeting may be unnecessary.