Skeletal muscle characteristics and metabolic response to exercise in young standardbreds.

OBJECTIVE: To determine whether performance capacity in a group of young trained Standardbreds is related to skeletal muscle characteristics and metabolic response to exercise. ANIMALS: 13 clinically normal 2-year-old Standardbreds. PROCEDURE: Venous blood and middle gluteal muscle biopsy samples were obtained from each horse within 1 to 2 minutes after trotting at high speed for 1,600 m. RESULTS: There was a positive correlation between plasma lactate and muscle glucose-6-phosphate (G-6-P) concentrations and trotting speed. There was a negative correlation between muscle adenosine triphosphate concentration and trotting speed, plasma lactate concentration, and muscle lactate and G-6-P concentrations. Muscle concentration of G-6-P was positively correlated with muscle lactate and plasma lactate concentrations. Percentage of type-1, -2A, and -2B fibers in skeletal muscle and muscle enzyme activities of hexokinase, 3-OH-acyl-CoA dehydrogenase, lactate dehydrogenase, and citrate synthase were not correlated with trotting speed. CONCLUSION: Speed of trotting at a distance of 1,600 m in young trained Standardbreds is related to the ability of muscle to produce energy by use of anaerobic glycolysis.