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1 Physiology and Biophysics, University of Alabama at Birmingham, Birmingham, Alabama, United States; Geriatric Research, Education, and Clinical Center, VA Medical Center, Birmingham, Alabama, United States
2 Surgery, University of Alabama at Birmingham, Birmingham, Alabama, United States
* To whom correspondence should be addressed. E-mail: mbamman{at}uab.edu.
Myostatin is a potent inhibitor of myogenesis; thus differential expression might be expected across individuals varying in responsiveness to myogenic stimuli. We hypothesized myostatin would be differentially regulated across humans with markedly different hypertrophic responses to resistance training (RT, 16 wk). Targets were assessed in muscle biopsies at baseline (T1) and 24 h after the first (T2) and last (T3) loading bouts in previously untrained subjects statistically clustered based on mean myofiber hypertrophy as extreme (Xtr, n=17, 2475 µm2), modest (Mod, n=32, 1111 µm2), and non-responders (Non, n=17, -16 µm2). We assessed protein levels of latent full-length myostatin protein complex (MSTNCMPLX) and its propeptide (MSTNPRO), mRNA levels of myostatin, cyclin D1, p21cip1, p27kip1, and activin receptor IIB, and serum myostatin protein concentration. Total RNA concentration increased by T3 in Non (37%) and Mod (40%), while it increased acutely (T2) only in Xtr (26%), remaining elevated at T3 (40%). Myostatin mRNA decreased at T2 (-44%) and remained suppressed at T3 (-52%) but not differentially across clusters. Cyclin D1 mRNA increased robustly by T2 (38%) and T3 (74%). The increase at T2 was driven by Xtr (62%) and Xtr had the largest elevation at T3 (82%). No effects were found for other target transcripts. MSTNCMPLX increased 44% by T3 but not differentially by cluster. MSTNPRO and circulating myostatin were not influenced by RT or cluster. Overall, we found no compelling evidence that myostatin is differentially regulated in humans demonstrating robust RT-mediated myofiber hypertrophy vs. those more resistant to growth.
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