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1 Institute of Sports Medicine, Bispebjerg Hospital, Copenhagen, Denmark
2 Department of Medical Biochemistry and Genetics, University of Copenhagen, Copenhagen, Denmark; Department of Molecular Muscle Biology, Copenhagen Muscle Research Centre, Rigshospitalet, Copenhagen, Denmark
3 Department of Physiology and Biophysics, University of California-Irvine, Irvine, California, United States
* To whom correspondence should be addressed. E-mail: katjaheinemeier{at}hotmail.com.
In skeletal muscle, an increased expression of insulin like growth factor-I isoforms - IGF-IEa and mechano growth factor (MGF) - combined with down-regulation of myostatin, is thought to be essential for training induced hypertrophy. However, the specific effects of different contraction types on regulation of these factors in muscle are unclear, and in tendon the functions of myostatin, IGF-IEa and MGF in relation to training are unknown. Female Sprague-Dawley rats were subjected to 4 days of concentric-, eccentric- or isometric training (n=7-9 per group) of the medial gastrocnemius, by stimulation of the sciatic nerve during general anaesthesia. mRNA levels for myostatin, IGF-IEa and MGF in muscle and Achilles tendon were measured by real-time RT-PCR. Muscle myostatin mRNA decreased in response to all types of training (2 to 8-fold) (p<0.05), but the effect of eccentric training was greater than concentric and isometric training (p<0.05). In tendon, myostatin mRNA was detected, but no changes were seen after exercise. IGF-IEa and MGF increased in muscle (up to 15-fold) and tendon (up to 4-fold) in response to training (p<0.01). In tendon no difference was seen between training types, but in muscle the effect of eccentric training was greater than concentric training for both IGF-IEa- and MGF (p<0.05), and for IGF-IEa isometric training had greater effect than concentric (p<0.05). The results indicate a possible role for IGF-IEa and MGF in adaptation of tendon to training, and the combined changes in myostatin and IGF-IEa/MGF expression, could explain the important effect of eccentric actions for muscle hypertrophy.
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