Abstract 2600: Mitochondria Dysfunction Precedes Cardiomyocyte Apoptosis in the Pressure-Overloaded Neonatal Left Ventricle
Objective: In pressure overload LV hypertrophy (LVH) cardiomyocyte apoptosis has been recognized as a major contributor to the progression from compensated hypertrophy to failure. We hypothesize that the mitochondrial apoptotic pathway is the likely source via electron transport chain (ETC) dysfunction and oxidative stress with resultant activation of the intrinsic mitochondrial pathway.
Methods: Neonatal rabbits underwent thoracic aortic banding at 10 days of age. Compensated hypertrophy (4wks), decompensated hypertrophy (6wks), and age-matched controls (n=4/group) were studied by serial echocardiography measuring LV mass/volume ratio (M/V) and fractional shortening (FS). ETC Complex I activities were determined by spectophotometry in isolated mitochondria (nmolesNADH/mg/min). Cardiomyocyte apoptosis (TUNEL staining) and Bcl-2/Bax ratios (immunoblotting) were measured. Data are expressed as median ± SEM with p≤.05 being significant by t-test.
Results: At 4wks M/V ratio was significantly increased (control 1.47±0.11 vs. banded 2.01±0.18, p=.009) and FS was preserved. Complex I activity was significant decreased (control 1,355±123 vs banded 949±25, p=.04), however no significant loss of cardiomyocytes had yet occurred. By 6wks M/V ratio was significantly decreased (LV dilation; control 1.67±0.13 vs banded 1.26±0.09, p=.01) and FS was impaired (39.2±1.3 vs 27.7±4.3, p=.016). Complex I activity was further decreased (Control 940±47 vs Banded 773±21, p=.03). Bcl-2/Bax ratios were significantly decreased (control 0.97±.002 vs banded 0.9±.003, p=.028) with a resultant significant increase in cardiomyotcyte apoptosis compared to 4wk banded animals (3.93±0.86 vs 13.03±4.19 TUNEL pos nuclei/1000 nuclei, p=.05).
Conclusion: In pressure overload hypertrophy, the transition from compensated LVH to failure and cardiomyocyte apoptosis is preceded by mitochondrial ETC dysfunction followed by a decrease in Bcl-2/Bax ratios. The likely mechanism is through increased reactive oxygen species generation and increased oxidative stress. Interventions to mitigate oxidative stress during compensated hypertrophy may prevent the onset of failure.