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

To determine whether hindlimb unloading (HU) alters the extracellular matri x 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 soleu s and plantaris muscles was examined biochemically for collagen abundance v ia measurement of hydroxyproline, and the percentage of cross-sectional are a of collagen was determined histologically with picrosirius red staining. Total hydroxyproline content in the soleus and plantaris muscles was unalte red by HU at any time point. However, the relative proportions of type I co llagen 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%). Corre spondingly, type III collagen increased in soleus muscle with 14 and 28 day s HU (Con 32 +/-5%; 14 days HU 47 +/-4%; 28 days HU 48 +/-7%). The proporti on 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 hyb rid 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 mu scle fiber composition in plantaris muscle. The data demonstrate that HU in duces 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.