Blood glucose clearance after feeding and exercise in polysaccharide storage myopathy

Polysaccharide storage myopathy (PSSM) in Quarter Horses (QH) and QH crosses is a glycogen storage disorder in which blood glucose clearance and insulin sensitivity, following an i.v. or oral glucose challenge, are enhanced. Exercise is known also to enhance glucose uptake into skeletal muscle in many animal species. Therefore, the purpose of this study was to compare the effect of exercise on glucose clearance in PSSM and control horses when an oral carbohydrate meal (8 Mcal sweet feed) was fed following either 12 h fasting alone (NEX protocol) or following fasting and a standard exercise protocol (EX protocol). In the NEX protocol, horses fasted 12 h and then were fed 8 megacalories (Mcal) of sweet feed (2.3 kg). In the EX protocol, horses were fed sweet feed 2 h after an exercise test (SET). Blood glucose was analysed for < 480 min after feeding. Muscle biopsies were taken before and after the EX protocol. With the NEX protocol, the mean of all blood glucose and insulin concentrations were significantly lower in fed-PSSM horses than controls (P < 0.013). The EX protocol in control horses caused a less pronounced percentage change in glucose concentration from baseline following feeding compared to the NEX protocol (mean peak EX 26.5% vs. NEX 70.2%, respectively) (P < 0.0003). In PSSM horses, the EX protocol also resulted in a lower percent change in glucose concentration following feeding compared to the NEX protocol (44.7 vs. 67.5%, respectively) (P = 0.021). The magnitude of the difference in percentage change of blood glucose concentration with the EX protocol was less in PSSM than that seen for controls (mean peak PSSM-EX 44.7% vs. 26.5% for controls, respectively) (P < 0.006). No significant differences in the insulin:glucose ratios were found for control horses between NEX and EX. In PSSM horses, the insulin:glucose ratio was significantly increased in the EX vs. NEX. In conclusion, exercise in normal horses results in enhanced glucose clearance following feeding without altering insulin:glucose ratios. At rest, PSSM horses have lower insulin:glucose ratios after feeding than normal horses, however, exercise in PSSM horses significantly increases insulin:glucose ratios. This may explain the beneficial effect of daily exercise for preventing rhabdomyolysis in horses with PSSM.