The effect of unloading and reloading on the extracellular matrix in skeletal muscle: changes in muscle strength and motor activity

During three weeks of hindlimb suspension muscle mass decreased 36%(p<0.05) in Soleus (Sol) muscle, 17%(p<0.05) in Gastrocnemius (GM) and had tendences to decrease in plantaris (Pla) (15%) and in extensor digitorum longus (EDL) (8%) muscles. Hindlimb grip strength decreased gradually during three weeks of unloading. Specific mRNA level for type I collagen decreased during three weeks of unloading in Sol muscle by 28%(p<0.05) and in GM muscle by 34%(p<0.05). mRNA level for type III collagen decreased in Sol by 22%(p<0.05) and in GM by 51%(p<0.001). Non-fibrillar type IV collagen mRNA level decreased in both above-described muscles about 25%(p<0.05). Lysyl oxidase (LO) mRNA level decreased by 46%(p<0.05) during three weeks of unloading only in Sol muscle. Matrix metalloproteinase-2 (MMP-2) mRNA level increased during reloading period in Sol and GM muscles subsequently 28%(p<0.05) and 49%(p<0.001). During unloading the activity of tissue inhibitors of metalloproteinases (TIMP-1 and TIMP-2) in slow-twitch (ST) and fast-twitch (FT) muscles changed in different directions: during first week of suspension, their expression decreased in Sol muscle by 31%(p<0.05) and increased in Pla and GM muscle subsequently by 24%(p<0.05) and 31%(p<0.001). The pretranslational level of changes in fibrillar and non-fibrillar collagen, MMP-2, LO, TIMP-1 and TIMP-2 -are shown for first time together with changes in muscle strength and motor activity during unloading and reloadin