It is well known that periods of contractile activity result in adaptations to muscle structure and function. Clearly, this type of muscle remodelling must involve changes in the rate of muscle protein turnover. One of the major pathways for muscle protein turnover is the calpain proteolytic system (Goll et al. 1998), which comprises several calpain isoenzymes and their specific endogenous inhibitor calpastatin. The calpain system is believed to be responsible for cytoskeletal remodelling in muscle fibres and is also associated with myoblast differentiation and fusion, fibre hypertrophy and growth. However, in human skeletal muscle little is known about the effects of exercise on the calpain system, and how this relates to changes in muscle composition and function.
Five female subjects (age 28 ± 2 years, body mass 69.0 ± 3.9 kg, height 1.69 ± 0.02 m) volunteered to participate in the study, which was approved by the local ethics committee. Supervised resistance training was performed on three occasions each week for a period of 8 weeks, followed by 4 weeks of detraining, where subjects were asked not to perform any strenuous activities. Needle muscle biopsies were obtained from the vastus lateralis (Bergstom, 1962) prior to training, following 8 weeks of training and after 4 weeks of detraining, and immediately frozen in liquid nitrogen. Total RNA was extracted from between 10 and 20 mg tissue, and the expression of calpastatin, m- and m-calpain and the muscle-specific p94-calpain determined by Taqman real-time PCR.
Calpastatin expression was significantly increased by 300 % following 8 weeks of training and remained upregulated 4 weeks after the cessation of training (P < 0.05). This was mirrored by m-calpain expression, which increased 154 % (P < 0.01) following training and remained elevated after training was terminated (P < 0.01). Training had no effect on either p94 or m-calpain expression, but both were significantly increased after 4 weeks of detraining (Table 1). Training induced significant increases in resistance workload. Four weeks of training produced a change in resistance from week 0 of 9 ± 2 kg for leg squat (P < 0.05) and 9 ± 1 kg for leg rowing (P < 0.05) exercises. These changes were magnified after 8 weeks of training: 20 ± 4 kg for leg squat (P < 0.05) and 14 ± 2 kg for leg rowing (P < 0.05) in addition to a resistance change of 19 ± 3 kg for leg press exercises (P < 0.05).
In conclusion, the data show that resistance training markedly increases the expression of m-calpain protease and to an even greater extent the calpain inhibitor, calpastatin. Furthermore, after detraining, m-calpain and calpastatin remained elevated, in addition to a significant upregulation of both muscle-specific p94 calpain and m-calpain protease. The implication of this induction of the calpain proteolytic system to muscle mass and fibre type is yet to be elucidated.
All procedures accord with current local guidelines and the Declaration of Helsinki.