Assessment of (patho)physiologic alterations in equine muscle metabolism

This thesis focussed on the diagnostic use of metabolic products and enzymes found in plasma, urine and muscle of the horse, the identification of which can reveal physiological or pathological changes in muscle metabolism. In this thesis analyses of carbohydrate-, lipid- and protein metabolites have been performed as well as analyses of the purine nucleotide cycle, the creatine metabolism and the oxidative phosphorylation. With these analyses we studied metabolic myopathies in the horse as well as metabolic effects of normal and intensified training and exercise. The application of these diagnostic techniques in treating horses with atypical myopathy revealed the pathogenesis of a new disorder of lipid metabolism in equines, namely acquired multiple acyl-CoA dehydrogenase deficiency (MADD), which appears to have a large and emerging prevalence in Europe and elsewhere. Several hypotheses for prevention and treatment of this disorder are presented in this thesis. The diagnostic analysis of creatine-, carbohydrate- and purine metabolism as well as oxidative phosphorylation was performed in healthy horses and horses with MADD. The goal of this study was to ascertain if, apart from the lipid metabolic disorder, the exogenic toxic insult associated with MADD disrupted any other metabolic cycles or reactions. Examples of general metabolic parameters for myopathic damage found included plasma creatine kinase (CK) and lactic acid, as well as urinary creatine and uric acid. Very low activity of the enzyme phosphoglycerate mutase (PGAM) and severely decreased activity of ATP-synthase were found in some patients. The effects of intense exercise and (intensified) training on protein (amino acids) and lipid (organic acids, carnitines) metabolism were studied. Ten Standardbred horses were trained for 32 weeks, divided into four phases, including a phase of intensified training for five horses. At the end of each phase, a standard exercise test (SET), was performed. Plasma amino acid, fatty acid and carnitine concentrations, before and after each SET, were measured. Training significantly reduced mean plasma aspartic acid concentration, whereas exercise significantly increased the plasma concentrations of alanine, taurine, methionine, leucine, tyrosine and phenylalanine and reduced the plasma concentrations of glycine, ornithine, glutamine, citrulline and serine. Concentrations of lactic acid, total nonesterified fatty acids, 3-OH-isobutyric acid and C2-carnitine were significantly increased following all SETS in exercised horses, whereas beta-hydroxybutyric acid, C3- and C4-carnitine were significantly decreased. No significant effect of training and intensified training was found. Our findings suggest that plasma acylcarnitine profile in the equine species might reflect skeletal muscle carnitine metabolism following exercise thereby providing a possible practical method to investigate potential disorders in carnitine metabolism in horses suffering from myopathy. A study to determine the effect of alfa-tocopherol acetate (vitamin E; 4 mg/kg BW/day) on glucose metabolism, peripheral insulin sensitivity and anti-oxidant status, showed that at this dose neither glucose metabolism nor peripheral insulin sensitivity were significantly altered. In addition, alfa-tocopherol acetate administration did not improve the measured parameters of antioxidant status.