Metabolic response of equine muscle to intermittent maximal exercise

Four thoroughbred horses performed 4 gallops (G1-G4) with intervals of 5 min. With one exception, gallops were sustained at maximal speed over 620 m. Muscle biopsy samples of the middle gluteal and brachiocephalicus were taken before, during, and after exercise and assayed for ATP and intermediary metabolites. The results showed a major involvement of the brachiocephalicus, in addition to the middle gluteal, during galloping. In three horses, who were clearly fatigued, muscle ATP decreased by up to 50% by the end of G4. This was matched by an equal rise in inosine 5'-monophosphate. Pronounced accumulations of glycerol 3-phosphate, glycerol, and lactate (up to 204 mmol X kg dry muscle-1) occurred with exercise. In the fourth horse, which was less fatigued, a decrease in ATP and increases in intermediary metabolites were much less. Postexercise there was little or no recovery in muscle ATP or lactate during 30 min. The decreases in ATP are consistent with a high activity of adenosine 5'-monophosphate deaminase in horse muscle and indicative also of the high level of anaerobic stress of the exercise program. There was evidence to suggest that the increase in muscle glycerol resulted from hydrolysis of glycerol 3-phosphate and not from the utilization of triglyceride.