Comparison of Skeletal Muscle Citrate Synthase Activity Across Equine Breeds

Skeletal muscle citrate synthase (CS) activity serves as a marker for muscle oxidative capacity. CS activity was compared among Quarter Horses (QH), Arabians (AR),  Thoroughbreds (TB), Standardbreds and Warmbloods at various ages and stages of training. Gluteus medius muscle biopsies were obtained at ages 6–23 m (n = 10), 2 y (n = 37), 3 y (n = 44) and >3 y (n = 55). Disciplines included reining (QH n = 17), ≥ 50-mile endurance (AR n = 20), racing (TB n = 47, STD n = 14) and dressage (WB n = 15). CS activity was measured fluorometrically. An unpaired t-test or oneway ANOVA compared age groups within breeds and age groups across breeds. A 2-way ANOVA compared breed and age effects across 2 and 3 y-old QH and TB, 3 and 4–7 y-old WB (insufficient numbers of 2 y-old AR and STD). CS was significantly lower in 2 y-old QH than either 2 y-old TB (43% higher, P < 0.0001) or 3 y-old WB (12%, P = 0.018). CS did not differ between 3 y-old QH and 3 y-old TB and was higher in 3 y-old TB than 4–7 y-old WB (P = 0.046). Within breeds, CS was lower in <2 versus 3 y in QH (44% lower, P = 0.001), 3–6 y in TB (35%, P = 0.02) and 6–19 y in AR (30%, P = 0.051) but not versus ≥5 y old WB (9%, P = 0.39). In all horses in full training, CS activity was not  significantly different across breeds (P = 0.099), with mean CS in QH being 20–24% lower versus TB and STD, 17% lower than AR and 4% lower than in WB, and wide individual variation apparent. In conclusion, in horses ≤2 y of age, differences exist in aerobic capacity between breeds. However, effects of age, training and individual  variation obscure breed differences in muscle CS activity in horses >3 y in training.

Iglewski, H., M. Henry, C. Fenger, J. Pagan, E. McKenzie, and S. Valberg. 2021. Comparison of skeletal muscle citrate synthase activity across equine breeds. Journal of Equine Veterinary Science 100:103608.