Non-exercising muscle metabolism during exercise

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Abstract
Glycogen decrements have been observed in non-exercising muscles during exercise. We therefore investigated whether the degraded glycogen was retained within the muscle in the form of glycolytic intermediates, or whether it was effluxed from the non-exercising muscles. For these studies a suspension harness was used to unload the hindlimb muscles at rest and during exercise [McDermott et al. (1987) J Appl Physiol 63:1275–1283]. Concentrations of glycogen and glycolytic intermediates glucose 6-phosphate, fructose 6-phosphate, fructose 1,6-bisphosphate, glycerol 3-phosphate, and lactate) were measured in non-exercising and exercising muscles (soleus, plantaris, red and white gastrocnemius) during a 90-min exercise bout 15 m/min, 8% grade). On-line electromyographic analysis showed that the contractile activity in the non-exercising muscles was markedly lower than in the exercising muscles. Similar decrements in muscle glycogen levels were observed in both the non-exercising and exercising muscles at the end of the 90-min, exercise bout (PP<0.05), although only slight changes in the glycolytic intermediates occurred. The sum total of all the accumulated glycolytic intermediates and lactate (converted to glucosyl units) in the non-exercising muscles only accounted for a small fraction of the glycogen degraded (≈ 15%–28%). We conclude that the metabolism of glycogen is enhanced in non-exercising muscle, and that glycogen utilization is uncoupled from the energetic demands of the muscle. Furthermore, the glycogen mobilized in non-exercising muscle is not retained within the muscle in other metabolite pools. We speculate that the carbon units derived from glycogen may be effluxed into the circulation to join the oxidizable/gluconeogenic carbon pool.