Increased IMP content in glycogen-depleted muscle fibres during submaximal exercise in man.

To study the relationship between glycogen depletion and IMP accumulation in different fibre types, single fibres were dissected from biopsies taken at rest and after one hour of exercise at 70% of maximal oxygen uptake. These fibres were analysed histochemically for glycogen and fibre types and pooled into classes of type I or type II fibres with low, medium or high glycogen content, in a total of six classes. These pools were analysed for ATP, ADP, AMP and IMP contents by high performance liquid chromatography. The contents of ATP, ADP and AMP at rest, and immediately after exercise, were not significantly different between the six fibre classes. The IMP content in glycogen-depleted fibres obtained after exercise was, however, higher than in pools of glycogen-filled fibres obtained both at rest and after exercise. In conclusion, the elevated IMP content in glycogen-depleted but not in glycogen-filled type I and type II muscle fibres during prolonged submaximal exercise indicates a decreased ATP regeneration rate in glycogen-depleted fibres, which may be a factor limiting exercise duration during prolonged submaximal exercise.