Metabolic changes in single human muscle fibres during brief maximal exercise

fibre type-dependent differences in high-energy phosphate (HEP) content at rest. Most often, type I fibres have been found to contain lower absolute amounts of phosphocreatine (PCr) and ATP compared with type II fibres (Söderlund et al. 1992; Greenhaff et al. 1994; Sant’Ana Pereira et al. 1996). Among pools of type II fibres an increasing resting PCr content has been reported as expression of myosin heavy chain (MyHC) IIX increased (Sant’Ana Pereira et al. 1996). Twenty-five seconds of maximal dynamic exercise resulted in almost total depletion of PCr in all fibre types and greater reductions in ATP content in type II fibre groups compared with type I fibres (Sant’Ana Pereira et al. 1996; Karatzaferi et al. 2001). It was concluded that changes in response to high intensity exercise varied according to the MyHC isoform content of the fibres (Sant’Ana Pereira et al. 1996). The observed low post-exercise ATP levels were accompanied by high IMP levels, which were higher for type II fibres, especially the those expressing large amounts of the IIX MyHC isoform (Sant’Ana Pereira et al. 1996). From the evidence of studies of stimulated amphibian fast muscle fibres (Nagesser et al. 1992), it could be suggested that accumulation of inosine-5fi-monophosphate (IMP), occurs only after PCr levels are depleted. A previous study in human muscle from our own laboratory also confirmed that IMP accumulation had occurred in muscle fibres that had been depleted of PCr (Sant’Ana Pereira et al. 1996). However, since after the 25 s dynamic exercise used in that study all fibre populations were depleted of PCr it was not possible to determine those fibres which were subjected to the greatest metabolic stress or the time course of events leading to IMP accumulation.