Effects of fasting on isolated murine skeletal muscle contractile function during acute hypoxia.

Stored muscle carbohydrate supply and energetic efficiency constrain muscle functional capacity during exercise and are influenced by common physiological variables (e.g. age, diet, and physical activity level). Whether these constraints affect overall functional capacity or the timing of muscle energetic failure during acute hypoxia is not known. We interrogated skeletal muscle contractile properties in two anatomically distinct rodent hindlimb muscles that have well characterized differences in energetic efficiency (locomotory- extensor digitorum longus (EDL) and postural- soleus muscles) following a 24 hour fasting period that resulted in substantially reduced muscle carbohydrate supply. 180 mins of acute hypoxia resulted in complete energetic failure in all muscles tested, indicated by: loss of force production, substantial reductions in total adenosine nucleotide pool intermediates, and increased adenosine nucleotide degradation product-inosine monophosphate (IMP). These changes occurred in the absence of apparent myofiber structural damage assessed histologically by both transverse section and whole mount. Fasting and the associated reduction of the available intracellular carbohydrate pool (~50% decrease in skeletal muscle) did not significantly alter the timing to muscle functional impairment or affect the overall force/work capacities of either muscle type. Fasting resulted in greater passive tension development in both muscle types, which may have implications for the design of pre-clinical studies involving optimal timing of reperfusion or administration of precision therapeutics.

Strain-Dependent Variation in Acute Ischemic Muscle Injury.

Limited efficacy of clinical interventions for peripheral arterial disease necessitates a better understanding of the environmental and genetic determinants of tissue pathology. Existing research has largely ignored the early skeletal muscle injury response during hind limb ischemia (HLI). We compared the hind limb muscle response, after 6 hours of ischemia, in two mouse strains that differ dramatically in their postischemic extended recovery: C57BL/6J and BALB/cJ. Perfusion, measured by laser Doppler and normalized to the control limb, differed only slightly between strains after HLI (<12% across all measures). Similar (<10%) effect sizes in lectin-perfused vessel area and no differences in tissue oxygen saturation measured by reflectance spectroscopy were also found. Muscles from both strains were functionally impaired after HLI, but greater muscle necrosis and loss of dystrophin-positive immunostaining were observed in BALB/cJ muscle compared with C57BL/6J. Muscle cell-specific dystrophin loss and reduced viability were also detected in additional models of ischemia that were independent of residual perfusion differences. Our results indicate that factors other than the completeness of ischemia alone (ie, background genetics) influence the magnitude of acute ischemic muscle injury. These findings may have implications for future development of therapeutic interventions for limb ischemia and for understanding the phasic etiology of chronic and acute ischemic muscle pathophysiology.