PULMONARY VO2 RESPONSE AND MUSCLE GLYCOGEN DEPLETION DURING REPEATED BOUTS OF HEAVY EXERCISE IN THE HORSE

Abstract

Prior heavy exercise facilitates the pulmonary VO2 response during the following ‘identical’ heavy exercise. While this mechanism is poorly understood, it may be linked to the cause of VO2 slow component. Whatever the mechanism, there is a general consensus that the VO2 slow component is attributable to factor(s) within the exercising muscle. The VO2 response to heavy exercise in the horse is known to be similar to that in humans. PURPOSE We investigated the temporal profile between pulmonary VO2 and muscle glycogen depletion pattern during repeated bouts of heavy exercise in the horse. METHODS Six Thoroughbred horses (4–6 yr) performed three protocols of heavy exercise at an intensity corresponding to 80% of VO2 max (running speed: 7.6–8.8 m/s) on a treadmill inclined at 10% slope: A) 1st bout (6 min) - baseline walking at 1.6 m/s (6 min) −2nd bout (6 min), B) 1st bout (6 min) - baseline walking (6 min) - 2nd bout (3 min), C) a single bout (3 min). VO2 and VCO2 were continuously measured and muscle biopsy samples were obtained from the gluteus medius muscle before and after each bout. Muscle fiber type composition was determined by ATPase staining and glycogen content was estimated by the PAS reaction. RESULTS The VO2 response to the 2nd bout showed a significantly faster adaptation compared to that in 1st bout during first minute after the onset of exercise, but the VO2 slow components of both bouts were similar. During the first 3 min of exercise bout, in the SO fibers, the muscle glycogen depletion of 2nd bout was significantly smaller than that of 1st bout. CONCLUSION The SO fibers might be used more anaerobically during early phase of 1st bout exercise compared to the 2nd bout, and such muscle glycogen depletion pattern might be associated with the facilitated VO2 adaptation during early phase after the onset of heavy exercise in the horse.