Structural And Functional Alterations In A Premature Ventricular Contractions-Induced Cardiomyopathy Model

Introduction: Previous studies of premature ventricular contractions (PVC)-induced cardiomyopathy (PVC-CM) in canines have showed reduction in L-type Ca 2+ current and Ca 2+ transient amplitude in isolated ventricular myocytes, concomitant with eccentric hypertrophy and a decline in left ventricular (LV) ejection fraction. Objectives: Assess potential alterations of PVC-CM related to: 1) the efficiency of the Ca 2+ recycling machinery in the sarcoplasmic reticulum (SR), and 2) the overall ultrastructural integrity of myocytes. Methods: Modified pacemakers were implanted in healthy canines to reproduce PVC-CM after 3 months of bigeminal PVC (50% burden at 200-220 ms coupling interval). Animals were randomized to either 50% PVC or no PVC (sham). Ventricular cardiomyocytes were isolated and paced to measure Ca 2+ transients using Fura2. Permeabilized cardiomyocytes loaded with Fluo-5N were used to assess SERCA2 activity in vitro . Additionally, expression of SERCA2, and phosphorylated versus total phospholamban (PLN) were measured with Western blot (WB). LV wall muscle was fixed, en-block stained with uranyl acetate, and thin-sectioned for electron microscopy (EM). Results: There was a non-statistically significant trend towards a reduction of SR-Ca 2+ reuptake efficiency in PVC-CM vs sham, evidenced by a 21% slower decay of the electrically-evoked cytosolic Ca 2+ transient and a 23% slower maximal rate of SR Ca 2+ uptake (PVC-CM n=42 cells, 4 animals vs sham n=50 cells, 3 animals). WB analysis showed a 23% reduction in SERCA2 expression (0.76 ± 0.18 vs 1 ± 0.25, P =0.002, n=6 animals each), a 40% increase in PLN expression (1.4 ± 0.18 vs 1 ± 0.1, P <0.0001, n= 3 animals each), and a 64% reduction in phospho-PLN (0.35 ± 0.15 vs 1 ± 0.18, P <0.0001, n=3 animals each) in PVC-CM vs sham. In addition, EM showed intact dyads, regularly shaped SR, but noticeable myofibril disorder in the PVC-CM derived samples. Conclusions: In PVC-CM, there is impaired electrically-evoked Ca 2+ cycling due to a combination of reduced L-type Ca 2+ channels and reduced SERCA-mediated SR Ca 2+ uptake. This combined with the apparent ultrastructural remodeling, underscore long-term cellular adaptations leading to LV systolic dysfunction associated to the chronic exposure to PVC.