Abstract
This study aimed at investigating the effects of 2, 4 and 8 weeks of endurance training on the contractile properties of slow (S), fast fatigue resistant (FR) and fast fatigable (FF) motor units (MUs) in rat medial gastrocnemius (MG) in relation to the changes in muscle mitochondrial biogenesis. The properties of functionally isolated MUs were examined in vivo. Mitochondrial biogenesis was judged based on the changes in mitochondrial DNA copy number (mtDNA), the content of the electron transport chain (ETC) proteins and PGC-1α in the MG. Moreover, the markers of mitochondria remodeling mitofusins (Mfn1, Mfn2) and dynamin-like protein (Opa1) were studied using qPCR. A proportion of FR MUs increased from 37.9% to 50.8% and a proportion of FF units decreased from 44.7% to 26.6% after 8 weeks of training. The increased fatigue resistance, shortened twitch duration, and increased ability to potentiate force were found as early as after 2 weeks of endurance training, predominantly in FR MUs. Moreover, just after 2 weeks of the training an enhancement of the mitochondrial network remodeling was present as judged by an increase in expression of Mfn1, Opa1 and an increase in PGC-1α in the slow part of MG. Interestingly, no signs of intensification of mitochondrial biogenesis assessed by ETC proteins content and mtDNA in slow and fast parts of gastrocnemius were found at this stage of the training. Nevertheless, after 8 weeks of training an increase in the ETC protein content was observed, but mainly in the slow part of gastrocnemius. Concluding, the functional changes in MUs’ contractile properties leading to the enhancement of muscle performance accompanied by an activation of signalling that controls the muscle mitochondrial network reorganisation and mitochondrial biogenesis belong to an early muscle adaptive responses that precede an increase in mitochondrial ETC protein content.
Highlights
Endurance training enhances muscle performance and resistance to fatigue [1,2,3,4,5]
Results of the present study indicate that functional changes in contractile properties in medial gastrocnemius (MG) muscle including an increase in fatigue resistance, shortening of the twitch time parameters and an increase in ability to potentiate force are evident after 2 weeks of training (Fig 3 and Table 1) but are restricted to fatigue resistant (FR) motor units (MUs)
The present study showed for the first time that as little as 2 weeks of endurance training is sufficient to induce pronounced changes in the contractile properties of MUs predominantly of the FR type
Summary
Endurance training enhances muscle performance and resistance to fatigue [1,2,3,4,5]. Prolonged periods of endurance training have been reported to modify the activity and content of proteins associated with ATP usage, such as sarco/endoplasmic reticulum (SR) Ca2+-ATPase (SERCA) pumps [6,7,8], parvalbumin and myosin heavy chain (MyHC) isoforms [9,10] as well as proteins involved in the ATP production system, including electron transport chain proteins in the mitochondria [3,11,12]. Meinild Lundby et al [17] have shown that six weeks of endurance training increases mitochondrial volume density (by about 40%) in humans, this training-induced augmentation in mitochondrial volume is a result of an enlargement of existing mitochondria and is not related to an increase in mitochondria content (de novo biogenesis)
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