Nitrogen metabolism in the mature physically conditioned horse

Abstract

Four mature geldings were subjected to a conditioning program consisting of galloping at daily workloads of 2.77 x 10('3) kg-km/d. Measurements including nitrogen balance, protein retention and skeletal muscle metabolism were taken prior to and following the conditioning program. Nitrogen balance increased from maintenance values as the horses were exercised. Protein concentration of wet muscle was minimally affected by conditioning. DNA concentration was not significantly affected, but there was an increase in RNA content of skeletal muscle in response to continuous exercise. Slow twitch, oxidative fibers increased in diameter in response to conditioning. Results indicate that protein synthesis capabilities of working skeletal musculature and oxidative capacity of existing musculature was increased due to conditioning. However, no evidence of large body composition changes in response to exercise was detected. Increased rate of water turnover during exercise suggests that sweat may be a large avenue for nitrogen loss, thus partially explaining higher nitrogen balance during exercise. Nitrogen intake was shown to affect nitrogen balance in exercising horses similarly to sedentary horses. Nitrogen balance increased with increased nitrogen intake and the relationship was characterized by the equation: Y-11.17 + .412X, where X is g of nitrogen intake and Y is the resulting nitrogen balance. All nitrogen balance values following conditioning were highly positive regardless of nitrogen intake, which suggests that nitrogen retention occurred. More research is needed to determine the significance of nitrogen loss in sweat so quantitative recommendations for protein requirements can be made for the exercising equine. Results of this study indicate that physical conditioning alters nitrogen metabolism in mature horses.