EFFECTS OF EXERCISE PRECONDITIONING ON MUSCLE HYPERTROPHY AND MITOCHONDRIAL REMODELING FOLLOWING THE SUBSEQUENT RESISTANCE TRAINING

Purpose: In response to resistance exercise training, it has been shown that individuals with a previous training history acquire muscle volume at an accelerated rate. This phenomenon may be attributed, in part, to the myonuclear enrichment resulting from the proliferation of muscle progenitor cells, which promotes essential protein synthesis following subsequent muscle training. As a highly energy demand tissue, the successful hypertrophy of muscle fiber depends on mitochondrial biogenic progression. Moreover, the majority of genes that encode mitochondrial proteins are within nuclear genome. Therefore, in this study, we investigated the effect of increased number of myonuclei in response to the previous resistance exercise preconditioning on mitochondrial adaptations to subsequent resistance training. Our central hypothesis was that pre-trained muscles would show an accelerated acquisition of training-induced mitochondrial function leading to a greater skeletal muscle hypertrophy compared to previously non-trained muscles and this may be associated with increased number of myonuclei in the pre trained muscles. Methods: Thirty-two Sprague-Dawley rats were randomly assigned to four groups (n=8 per group) which include control, pre-training, training, and retraining group. Resistance exercise training was carried out by ladder climbing with weights attached to the tail at ages of either 8- (pre-training) and 36-week-old (training), or both (retraining). Each training session consisted of 3 sets of 5 repetitions, and the training protocol was performed every third day for 8 weeks. At 44 weeks of age, specific muscle groups were carefully collected and stored at -80 °C until further analyses. 4', 6-Diamidino-2-phenylindole staining, hematoxylin & eosin staining, cytochrome c oxidase and succinate dehydrogenase staining were performed. Western blotting and immunohistochemstry were performed to assess the abundance of mitochondrial regulatory proteins and the mitochondrial content. In complementary in vitro studies, confluent L6 myoblast cells were further grown in differentiation media for 4 days with or without insulin-like growth factor 1 (50 ng/ml) supplementation. Mitochondrial gene expression levels and mitochondrial respiratory function were assessed after 5-Aminoimidazole-4-carboxamide ribonucleotide (AICAR, 1 mM), a 5' AMP-activated protein kinase activator, treatment. Results: Myonuclei numbers were higher in training and retraining groups than control group (all, p < 0.05), suggesting that ladder climbing training protocol increased myonuclei number. There was a significantly higher level of myonuclei number in pretraining group compared to the control group indicating that the acquired myonuclei during exercise preconditioning were retained over the 20-week detraining period. Muscle cross-sectional area, mitochondrial content and mitochondrial enzymatic activities (COX and SDH) were significantly greater in retraining group compared to training group (p < 0.01, p < 0.01 and p < 0.05, respectively). In in vitro study, L6 myotubes preconditioned with IGF-1 showed increased myonuclei numbers within each myotube and presented a higher level of mitochondrial gene expression and oxygen consumption rate under AICAR treatment condition. Conclusions: These data provide physiological evidence that pre-trained muscle with more myonuclei make the muscles more responsive to subsequent training in terms of muscle hypertrophy and mitochondrial remodeling. Furthermore, this study provides a proof-of-concept of biological processes underlying potential nuclear-mitochondrial interplay during muscle hypertrophy. These findings warrant future studies to identify a novel target for mitochondrial medicine to treat muscle atrophy.

Grantor dc:publisher

Temple University. Libraries

Year dc:date.issued

2016

Author dc:creator

• Lee, Hojun

Advisor dc:contributor.advisor

• Park, Joon Young

Committee members dc:contributor.committeemember

• Park, Joon Young
• Tierney, Ryan T.
• Hudson, Matthew Bryant
• Chen, Xiongwen

• Kinesiology
• Hypertrophy
• Mitochondrial remodeling
• Myonuclei
• Resistance exercise

Statement dc:rights

• IN COPYRIGHT- This Rights Statement can be used for an Item that is in copyright. Using this statement implies that the organization making this Item available has determined that the Item is in copyright and either is the rights-holder, has obtained permission from the rights-holder(s) to make their Work(s) available, or makes the Item available under an exception or limitation to copyright (including Fair Use) that entitles it to make the Item available.

Licence dc:rights.uri

• http://rightsstatements.org/vocab/InC/1.0/

Language dc:language.iso

eng

Dc Identifier Other

965642555

OAI identifier oai:identifier

oai:scholarshare.temple.edu:20.500.12613/1706