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1 Department of Anatomy, University of Bern, Bern, Switzerland
* To whom correspondence should be addressed. E-mail: daepp{at}ana.unibe.ch.
This study investigated the use of the hindlimb suspension (HS) and reloading model of mice for the mapping of ultrastructural and gene expressional alterations underlying loaddependent muscular adaptations. Mice were hindlimb suspended for seven days or kept as controls (C, n=12). Soleus muscles were harvested after HS (HS7, n=23) or after resuming ambulatory cage activity (reloading) for either one (R1, n=13) or seven days (R7, n=9). Using
electron microscopy, a reduction in mean fiber area (-37%) and in capillary-to-fiber ratio (from 1.83 to 1.42) was found for HS7. Subsequent reloading caused an increase in
interstitial cells (+96%) and in total capillary length (+57%) while mean fiber area and capillary-to-fiber ratio did not significantly change compared to HS. Total RNA in the soleus muscle was altered with both HS (-63%) and reloading (+108% in R7 compared to C). This is seen as an important adaptive mechanism. Gene expression alterations were assessed by a muscle-specific low-density cDNA microarray. The transcriptional adjustments indicate an
early increase of myogenic factors during reloading together with an overshoot of contractile (MyHC I and IIa) and metabolic (glycolytic and oxidative) mRNA amounts and suggest mechano-sensitivity of factors keeping the sarcomeres in register (desmin, titin, integrin 
1).
Important differences to published data from former rat studies were found with the mouse HS model for contractile and glycolytic enzyme expression. These species-specific
differences need to be considered when transgenic mice are used for the elucidation of monogenetic factors in mechano-dependent muscle plasticity.
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