Mitochondrial dysfunction in pulmonary hypertension syndrome in broiler chickens

The studies conducted in this dissertation were designed to investigate liver mitochondrial function in broiler chickens with and without Pulmonary Hypertension Syndrome (PHS, ascites). The purpose of the first study (Chapter 2) was to assess mitochondrial function and a mitochondrial antioxidant, glutathione (GSH) in response to oxidative stress in mitochondria in vitro. Liver mitochondria from Control and PHS broilers were incubated with 0, 1, and 5-mM tertiary-butyl hydroperoxide (tBH). The results indicate that PHS mitochondria do not lack antioxidant protection from GSH, but lower respiratory control ratio (RCR) and ADP:O ratios in PHS mitochondria indicate a dysfunction that may contribute to the pathophysiology of this metabolic disease in broilers. In Chapter 3, a series of chemical inhibitors of mitochondrial respiration were used to identify a site-specific defect in the electron transport chain in mitochondria obtained from broilers with PHS. Located at the succinate:ubiquinone oxidoreductase (Complex II/CoQ interface, this defect would allow electrons to leak from the respiratory chain and consume O₂ by forming reactive oxygen species at a greater rate than in Control mitochondria. Respiration studies revealed differences in the RCR as well as ADP:O ratio (an index of oxidative phosphorylation) between Control and PHS mitochondria that were accentuated by sequential additions of ADP to isolated mitochondria. Interestingly, results of sequential additions on liver mitochondria isolated from Control broilers and single comb white leghorns, that are resistant to developing PHS, were nearly identical. The findings in Chapter 4 provide further evidence and characterization of the altered cellular O₂ utilization in broilers with PHS. Proton conductance (a primary contributor of State 4 respiration) was assessed by simultaneously measuring O₂ consumption and mitochondrial membrane potential. The relationship of curves depicting State 4 respiration and mitochondrial membrane potential indicates that PHS mitochondria exhibit both impaired substrate oxidation and reduced proton conductance relative to Controls.