Expression of Titin in Skeletal Muscle Varies with Hind-Limb Unloading

The effects of prolonged hind-limb unloading on titin antibody localization and expression of titin isozymes of single fibers from the synergistic slow-twitch soleus (SOL) and fast-twitch plantaris (PLN) of adult rats were studied after 14 and 28 days of hind-limb unloading (HU). Titin antibody localization and expression was not altered at 14 days of HU. However, there was a 4% loss in antibody to Z-band distance (Ab-Z) in the SOL and an increase of 8% in PLN Ab-Z after 28 days of HU. The titin and myosin heavy chain composition of single fibers and small bundles of fibers from control and unloaded muscles were examined using 2% to 12% sodium dodecyl sulfate-polyacrylamide gel electrophoresis. There was a marked loss of relative amounts of titin in both SOL and PLN following 28 days of HU. As the protein loads for these measures were identical, the authors conclude that these findings represent an actual loss of titin density rather than a decreased value due to a loss of total muscle mass. Laser scanning densitometry of the titin bands show a marked decrease in density and molecular weight in unloaded SOL. In the PLN, marked losses of titin density were accompanied by decreased electrophoretic motility. The results demonstrate that the titin isoform composition and titin antibody localization of skeletal muscle is altered during hind-limb unloading. Furthermore, as titin is responsible for positional stability of the sarcomere and the fiber during contraction, change in isoforms during HU may predispose atrophied muscle to injury during reuse and recovery.