Hindlimb unloading induces a collagen isoform shift in the soleus muscle of the rat

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Abstract
To determine whether hindlimb unloading (HU) alters the extracellular matrix of skeletal muscle, male Sprague-Dawley rats were subjected to 0 (n = 11), 1 (n= 11), 14 (n = 13), or 28 (n = 11) days of unloading. Remodeling of the soleus and plantaris muscles was examined biochemically for collagen abundance via measurement of hydroxyproline, and the percentage of cross-sectional area of collagen was determined histologically with picrosirius red staining. Total hydroxyproline content in the soleus and plantaris muscles was unaltered by HU at any time point. However, the relative proportions of type I collagen in the soleus muscle decreased relative to control (Con) with 14 and 28 days HU (Con 68 ± 5%; 14 days HU 53 ± 4%; 28 days HU 53 ± 7%). Correspondingly, type III collagen increased in soleus muscle with 14 and 28 days HU (Con 32 ± 5%; 14 days HU 47 ± 4%; 28 days HU 48 ± 7%). The proportion of type I muscle fibers in soleus muscle was diminished with HU (Con 96 ± 2%; 14 days HU 86 ± 1%; 28 days HU 83 ± 1%), and the proportion of hybrid type I/IIB fibers increased (Con 0%; 14 days HU 8 ± 2%; 28 days HU 14 ± 2%). HU had no effect on the proportion of type I and III collagen or muscle fiber composition in plantaris muscle. The data demonstrate that HU induces a shift in the relative proportion of collagen isoform (type I to III) in the antigravity soleus muscle, which occurs concomitantly with a slow-to-fast myofiber transformation.