Nitrogen Metabolites During Exercise in the Horse

Abstract

In experiment one, eight Quarter Horse mares were intravenously infused prior to exercise with either sodium acetate (control) or ammonium acetate to determine the effect of elevated ammonia (NH$\sb3$) levels on fatigue development and muscle metabolism. The horses were exercised on an 11% grade treadmill at 4.4 m/s, carrying 27 kg of lead. Time to fatigue was not (P $>$.05) different between groups. Middle gluteal muscle (im) NH$\sb3$ and lactate increased (P $$.05). Treatment did not (P $>$.05) affect plasma or im ala, glu or gln. Urinary orotic acid/creatinine ratio was not (P $>$.05) affected by exercise or treatment and suggests that the urea cycle was not impaired during exercise. These results did not show any metabolic evidence for a role of increased blood NH$\sb3$ in fatigue development at the level of the central nervous system.

In a second experiment, six mature Quarter Horse mares were used in a crossover design to assess the effect of dietary protein level on energy and nitrogen metabolism during exercise. After a 2-wk adaptation to either a 9.0% (control) or an 18.5% crude protein (high-protein: HP) diet, each mare performed an exercise test (ET). The ET was performed in the absorptive stage of digestion and consisted of a 15 min exercise period at 4.5 m/s on an 11% grade treadmill. The dietary treatment did not (P $>$.05) affect heart rate or oxygen uptake; however, exercise resulted in an increase (P $$.05) effects due to treatment. A diet by exercise interaction (P $<$.01) occurred for plasma ala, being lower in the HP horses. Plasma urea-N, the urinary urea-N/creatinine and orotic acid/creatinine ratio were higher (P $<$.001) in the HP group. The excretion of urea-N in sweat was also higher (P $<$.001) in the HP group. Substrate utilization during exercise was not altered by excess dietary protein; however, excess protein stimulated nitrogen metabolism in the horse at rest, during submaximal exercise and during recovery.