Blood chemistry and skeletal muscle metabolic responses to exercise in horses with recurrent exertional rhabdomyolysis

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Abstract
Six horses with a history of recurrent exertional rhabdomyolysis (RER) and 7 control horses performed both a 55-min submaximal and later a 10-min near-maximal exercise test on a treadmill. Blood samples were obtained during exercise and gluteus medius muscle biopsies were obtained before and immediately after each exercise test and at 24 h after completion of the submaximal test. Rhabdomyolysis was developed by 3 of 6 RER horses during submaximal exercise and in 1 of the RER horses during near-maximal exercise. Concentrations of potassium, glucose, free fatty acids, ammonia, lactate, cortisol, adrenaline and noradrenaline in the blood were measured. None of these variables appeared useful in predicting which RER horses would develop rhabdomyolysis. The RER horses that developed rhabdomyolysis (RERa) had higher cortisol and blood glucose concentrations but otherwise had blood chemistry and muscle metabolic responses during submaximal and near-maximal exercise similar to those of RER horses which did not develop rhabdomyolysis (RERb) and to controls. At rest, muscle glycogen concentrations were significantly higher (> 650 mmol/kg dry wt) in RERa and RERb horses than in controls. Lactate concentrations in muscle after submaximal and near-maximal exercise were similar or lower, respectively, in RERa horses compared with controls. The results of this study indicate that, although horses with RER had high resting intramuscular glycogen concentrations, rhabdomyolysis did not appear to be caused by an excessively rapid rate of anaerobic glycolysis with lactate accumulation.