Biochemical basis and developmental aspects of polysaccharide storage myopathy in quarter horses.

摘要

Quarter Horses with Polysaccharide Storage Myopathy (PSSM) accumulate excessive amounts of glycogen and an abnormal polysaccharide in their skeletal muscle fibers, without an obvious defect in muscle glycolysis.; The purpose of this study was to determine if excessive glycogen accumulation in PSSM horses is due to enhanced cellular glucose uptake using an intravenous (IVGTT) and oral glucose tolerance tests (OGTT), and a modified Insulin Tolerance Test (MITT). PSSM horse blood glucose concentration was significantly lower than control horses in the fasted, but not in the non-fasted state. Glucose clearance rate of PSSM horses in the IVGTT was 2%/min compared to controls at 1.3%/min., and the MITT in PSSM horses resulted in a sustained hypoglycemia for more than 3 hours whereas glucose concentration returned to normal within 2 hours in controls. We concluded that PSSM Quarter Horses have a glycogen storage disorder in which blood glucose clearance and insulin sensitivity are enhanced. Therefore, the increased glycogen accumulation seen in QH with PSSM seems to be the result of increased synthesis due to increased insulin sensitivity.; Exercise is also known to enhance glucose uptake into skeletal muscle in many animal species, so the purpose of this second study was to test the hypothesis that exercise would further increase the rate of glucose clearance in PSSM and increase the rate of glycogen synthesis. This second experiment on glucose tolerance tests using carbohydrate at rest and after a bout of treadmill exercise showed that the insulin:glucose ratio following feeding could be similar to Control horses when PSSM horses are exercised prior to a meal. This may explain one of the beneficial effects of daily exercise for PSSM horses.; The final investigation suggests a heritable basis for PSSM since 3 out of 4 foals born to PSSM mares and followed until 2 years of age developed rhabdomyolysis and abnormal polysaccharide accumulation in skeletal muscle. By 6 months of age PSSM foals have evidence of rhabdomyolysis and enhanced insulin sensitivity. Abnormal PAS positive inclusions develop slowly over time (>12 months in two foals) confounding a diagnosis of PSSM in young foals with rhabdomyolysis.