Journal of Applied Physiology Journal of Neurophysiology
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J Appl Physiol 103: 1915, 2007; doi:10.1152/japplphysiol.00946.2007
8750-7587/07 $8.00
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LETTER TO THE EDITOR

Reply to Dr. Heinemeier

TO THE EDITOR: Dr. Heinemeier (3) questioned whether the declining myostatin mRNA levels we reported in Kim et al. (5) were in fact the result of increasing 18S ribosomal RNA (the normalizing denominator) rather than actual reductions in myostatin mRNA. As highlighted in our paper, resistance exercise-mediated decrements in myostatin mRNA are commonly reported in the literature, irrespective of the PCR and/or normalization procedure employed. Decreases have been found in rodents with the use of techniques similar to ours (2) and in humans via real-time quantitative PCR using various means of normalization including GAPDH (7) and 18S (4) with unchanging Ct values. We reanalyzed our data in preparation for this reply and found that myostatin mRNA persistently decreased with resistance exercise (both acute and long term) without 18S normalization, as noted for the raw data and adjusted per milligram muscle mass. We are therefore confident that a reduction in myostatin mRNA is a real result of resistance exercise. If changes in target mRNAs were strictly driven by increases in 18S across training, we would not expect to find significant increases in other target transcripts as reported for cyclin D1 in the paper by Kim et al. (5) and for numerous mRNAs from this same cluster model in a recent report (1). Lack of agreement between changes in myostatin mRNA and myostatin protein should not be surprising based on the growing complexity of regulatory processes controlling mRNA fates and translation initiation processes and we refer Dr. Heinemeier to a recent review on the topic by Moore (6). As stated above, we are not the first to report declining myostatin mRNA expression with resistance exercise; thus the emphasis of Kim et al. (5) is on the novel finding that changes in myostatin mRNA and various myostatin protein components do not differ among response clusters. The results and subsequent discussion strongly indicate that myostatin levels, whether protein or mRNA, whether increasing or decreasing, do not play a pivotal role in determining hypertrophy efficacy during resistance training in normal, healthy human volunteers.

FOOTNOTES


Address for reprint requests and other correspondence: M. M. Bamman, 966 McCallum Bldg., 1530 3rd Ave. South, Birmingham, AL 35294-0005 (e-mail: mbamman{at}uab.edu)

REFERENCES

  1. Bamman MM, Petrella JK, Kim JS, Mayhew DL, Cross JM. Cluster analysis tests the importance of myogenic gene expression during myofiber hypertrophy in humans. J Appl Physiol 102: 2232–2239, 2007.[Abstract/Free Full Text]
  2. Haddad F, Adams GR. Aging-sensitive cellular and molecular mechanisms associated with skeletal muscle hypertrophy. J Appl Physiol 100: 1188–1203, 2006.[Abstract/Free Full Text]
  3. Heinemeier KM. Using ribosomal RNA as a reference in mRNA quantification. J Appl Physiol; doi:10.1152/japplphysiol.00862.2007.
  4. Jones SW, Hill RJ, Krasney PA, O'Conner B, Peirce N, Greenhaff PL. Disuse atrophy and exercise rehabilitation in humans profoundly affects the expression of genes associated with the regulation of skeletal muscle mass. FASEB J 18: 1025–1027, 2004.[Abstract/Free Full Text]
  5. Kim JS, Petrella JK, Cross JM, Bamman MM. Load-mediated downregulation of myostatin mRNA is not sufficient to promote myofiber hypertrophy in humans: a cluster analysis. J Appl Physiol (August 2, 2007). doi:10.1152/japplphysiol.01194.2006.
  6. Moore MJ. From birth to death: the complex lives of eukaryotic mRNAs. Science 309: 1514–1518, 2005.[Abstract/Free Full Text]
  7. Raue U, Slivka D, Jemiolo B, Hollon C, Trappe S. Myogenic gene expression at rest and after a bout of resistance exercise in young (18–30 yr) and old (80–89 yr) women. J Appl Physiol 101: 53–59, 2006.[Abstract/Free Full Text]

Marcas M. Bamman
 Department of Physiology and Biophysics, University of Alabama at Birmingham, Birmingham, Alabama





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