Skeletal muscle mitochondrial myopathy as a cause of exercise intolerance in a horse.

Although exertional myopathies are commonly recognized in horses, specific etiologies have not been identified. This is the first report in the horse of a deficiency of Complex I respiratory chain enzyme associated with profound exercise intolerance. Physical examination, routine blood tests, endoscopy, and ultrasonograms of the heart and iliac arteries were unremarkable. With slow, incremental exercise (speeds 1.5-7 m/s), the Arabian mare showed a marked lactic acidosis, increased mixed venous PVO2, and little change in oxygen consumption. Muscle biopsies contained large accumulations of mitochondria with bizarre cristae formations. Biochemical analyses revealed a very low activity of the first enzyme complex in the mitochondrial respiratory chain (NADH CoQ reductase). The exercise intolerance and muscle stiffness in this horse were attributed to a profound lactic acidosis resulting from impaired oxidative energy metabolism during exercise.