Metabolic response to high-intensity exercise in the thoroughbred horse
The Thoroughbred racehorse is capable of maintaining speeds of approximately 17 m/s for distances of over a mile. This represents an average speed and the Thoroughbred can reach speeds in excess of 20 m/s over short distances. The present series of studies was undertaken to investigate further the metabolic response to high-intensity exercise in the Thoroughbred racehorse. Unlike man, high-intensity exercise in the horse results in an increase in packed cell volume. This in turn causes acute changes in the colligative properties of blood and plasma. The changes in these properties were investigated and the effect on calculation and distribution of metabolites in blood and muscle was determined. The horse has a high capacity for lactate production compared to man and existing methodology for the assessment of muscle buffering capacity in biopsy samples was further developed and investigated. The horse was found to have a significantly higher muscle buffering capacity compared with man and it was calculated that this could be explained wholly on the basis of the higher muscle carnosine content in the horse. The metabolic response to high-intensity exercise was investigated using several exercise models, including single and multiple field gallops and treadmill exercise. A consistent finding was that high-intensity exercise in the Thoroughbred racehorse was nearly always accompanied by a reduction in muscle ATP content. The nature of the ATP decrease was further investigated using a treadmill exercise model. Muscle ATP decrease was found to occur at a particular exercise intensity rather than show a gradual decrease with increasing intensity. The intensity at which muscle ATP content began to decline significantly varied between individual horses, but in each case appeared to coincide with muscle lactate contents of approximately 70 mmol/kg dry muscle. The significance of the decline in ATP is discussed.