Alterations of sarcoplasmic reticulum-mediated Ca uptake in a model of premature ventricular contraction (PVC)-induced cardiomyopathy.

Premature ventricular contractions (PVCs) are the most frequent ventricular arrhythmias in the overall population. PVCs are known to acutely enhance contractility by the post-extrasystolic potentiation phenomenon, but over time persistent PVCs promote PVC-induced cardiomyopathy (PVC-CM), characterized by a reduction of the left ventricular (LV) ejection fraction. Ca cycling in myocytes commands muscle contraction and in this process, SERCA2 leads the Ca reuptake into the sarcoplasmic reticulum (SR) shaping cytosolic Ca signal decay and muscle relaxation. Altered Ca reuptake can contribute to the contractile dysfunction observed in PVC-CM. To better understand Ca handling using our PVC-CM model (canines with 50% PVC burden for 12 weeks), SR-Ca reuptake was investigated by measuring Ca dynamics and analyzing protein expression. Kinetic analysis of Ca reuptake in electrically paced myocytes showed a ~ 21 ms delay in PVC-CM compared to Sham in intact isolated myocytes, along with a ~ 13% reduction in SERCA2 activity assessed in permeabilized myocytes. Although these trends were not statistically significant between groups using hierarchical statistics, relaxation of myocytes following contraction was significantly slower in PVC-CM vs Sham myocytes. Western blot analyses indicate a 22% reduction in SERCA2 expression, a 23% increase in phospholamban (PLN) expression, and a 50% reduction in PLN phosphorylation in PVC-CM samples vs Sham. Computational analysis simulating a 20% decrease in SR-Ca reuptake resulted in a ~ 22 ms delay in Ca signal decay, consistent with the experimental result described above. In conclusion, SERCA2 and PLB alterations described above have a modest contribution to functional adaptations observed in PVC-CM.