BIOCHEMICAL PROFILE OF THE GLUTEUS MEDIUS MUSCLE IN HORSES WITH DIFFERENT PERFORMANCE RECORDS

Biopsies from three different depths of the Gluteus medius muscle were obtained from 36 endurance horses, aged 8.42 +/- 2.85 years and of bo th sexes. Twenty of the horses were considered excellent performers ac cording to the mean speed of their three fastest records in endurance events, two or three years prior to the study. The other 16 horses wer e considered moderate performers, with a mean racing speed <12.5 km/h (in 120- to 180-km endurance rides). <14 km/h (in 80- to 120-km endura nce rides), or <13.5 km/h (in 40- to 60-km endurance rides). The activ ities of citrate synthase (an indicator of the citric acid cycle activ ity), 3-OH-acyl-CoA-dehydrogenase (a marker for lipid oxidation) and l actate dehydrogenase (an indicator of anaerobic metabolism) enzymes we re also studied. Significant differences in oxidative enzyme activitie s but not in the glycolytic lactate dehydrogenase enzyme activities we re recorded between performance categories. Excellent performers had h igher activities of citrate synthase (P<0.001) and 3-OH-acyl-CaA-dehyd rogenase (P<0.02) enzymes than the poorest. The three enzyme activitie s changed significantly with the increase of sampling depth within the muscle Citrate synthase and 3-OH-acyl-CoA-dehydrogenase activities in creased by 60% and 75%, respectively, and that of lactate dehydrogenas e decreased by 23% from surface to deep sampling depths. When lactate dehydrogenase/citrate synthase and lactate dehydrogenase/3-OH-acyl-CoA -dehydrogenase enzyme activity ratios were regressed on sampling depth in order to describe the general metabolic profile of the muscle, a s trong linear relationship was found in both performance groups (P<0.00 1). The highest intercepts of these regression lines, found in moderat e performers when compared to excellent performers (P<0.001 and P<0.01 for both ratios), clearly show that the best performing endurance hor ses have a,greater aerobic capacity and a relatively lower anaerobic c apacity in the Gluteus medius muscle than those horses that have been moderately endurance raced. These results show that fibre metabolic pr ofile of this muscle is directly related to the athletic ability of th e horse for endurance events.