Moderate and high intensity sprint exercise induce differential responses in COX4I2 and PDK4 gene expression in Thoroughbred horse skeletal muscle



Reasons for performing study: The role of molecular signalling pathways in the phenotypic adaptation of skeletal muscle to different exercise stimuli in the Thoroughbred horse has not been reported previously.

Objective: To examine CKM, COX4I1, COX4I2 and PDK4 gene expression following high intensity sprint and moderate intensity treadmill exercise stimuli in skeletal muscle of Thoroughbred horses.

Materials and methods: Two groups of trained 3-year-old Thoroughbred horses participated. Group A (n = 6 females, n = 3 males) participated in an incremental step test (moderate intensity) to fatigue or HRmax on a Sato high speed treadmill (distance = 5418.67m ± 343.21). Group B (n = 8 females) participated in routine ‘work’ (sprint) on an all-weather gallop (distance = 812.83 m ± 12.53). Biopsy samples were obtained from the gluteus medius pre-exercise (T0), immediately post exercise (T1) and 4 h post exercise (T2). For physiological relevance venous blood samples were collected to measure plasma lactate and creatine kinase concentrations. Changes in mRNA expression were determined by quantitative real-time RT-PCR for creatine kinase muscle (CKM), cytochrome c oxidase subunit IV isoform 1 (COX4I1), cytochrome c oxidase subunit IV isoform 2 (COX4I2) and pyruvate dehydrogenase kinase, isozyme 4 (PDK4) genes. Statistical significance (α<0.05) was determined using Student's t tests.

Results: COX4I2 mRNA expression decreased significantly in Group A and remained unchanged in Group B between T0 vs. T2 (−1.7-fold, P = 0.017; −1.0-fold, P = 0.859). PDK4 mRNA expression increased significantly in Group B but not in Group A between T0 vs. T1 (3.8-fold, P = 0.039; 1.4-fold, P = 0.591). There were no significant changes in the expression in CKM or COX4I1 mRNA abundance in either group.

Conclusions: Different exercise protocols elicit variable transcriptional responses in key exercise relevant genes in equine skeletal muscle due to variation in metabolic demand.