Abstract
Mitochondria are critical for oxidative phosphorylation in skeletal muscle, especially in athletic species such as the horse. Mitochondrial respiration increases with physical exercise, but the relationship between mitochondrial respiration and cardiovascular functions are not well described in the horse. The objective of this study was to determine if there is a relationship between heart rate (HR) during and after submaximal exercise tests (SETs) and skeletal muscle mitochondrial respiration in polo ponies. We hypothesized that horses with greater maximum HR and average HR during the exercise tests would have greater mitochondrial respiration in skeletal muscle. Twelve fit polo ponies (14.8 ± 1.7 years old, 10 mares and 2 geldings) were equipped with Polar equine heart rate monitors (Polar Electro Inc., Lake Success, NY) and underwent 26-minute SETs designed to mimic a polo chukker followed by a 30-minute recovery period. Muscle biopsy samples from the semitendinosus muscle were taken 2 weeks prior to the SET to determine mitochondrial oxygen consumption using the Oroboros O2k high-resolution respirometer (Oroboros Instruments, Innsbruck, Austria). Data were analyzed using the PROC CORR procedure (SAS Inst. Inc., Cary, NC). Correlations were considered strong at r > 0.6 and significant at P < 0.05. Maximum HR during SET and state IV respiration were positively correlated (P = 0.02, r = 0.68). Average HR during SET and state IV respiration were also positively correlated (P = 0.01, r = 0.72). However, correlations between maximum and average HR and state III respiration were not as strong (P ≥ 0.05, r < 0.6). These data suggest that state IV mitochondrial respiration (proton leak) in equine skeletal muscle may impact cardiac responses to high-intensity exercise. Horses with higher HR during exercise may have less efficient oxidative phosphorylation, resulting in earlier fatigue and/or greater formation of reactive oxygen species resulting from proton leak.