抄録
We recently reported the effects of long-term hindlimb immobilization (6 weeks) on the contractile properties of the soleus muscle in the rat (Annual meeting of the Physiological Society of Japan, 2007). We found that long-term hindlimb immobilization lowers the expression of the giant protein titin in the soleus muscle, resulting in reduced active force production via abnormal sarcomeric organization. In the present study, we investigated how immobilization affected fatigability by using single skinned fibers taken from the same animal model. Hydrogen ion (H+) and inorganic phosphate (Pi) are the well-known major causes of muscular fatigue. The intracellular concentrations of Pi and H+ are known to increase in skeletal muscle during intense exercise, resulting in a fall in active force production. Accordingly, we tested the effects of changes in pH and the Pi concentration on maximal Ca2+-activated force production in control vs. immobilized fibers. We found that lowering pH from 7.0 to 6.2 decreased maximal force in both muscles, with a greater magnitude in immobilized fibers. Likewise, the inhibitory effect of Pi up to 20 mM was more pronounced in immobilized fibers. These results suggest that fatigability is enhanced in disused muscle and that the mechanism includes a decrease in the fraction of force-generating cross-bridges coupled with abnormal sarcomeric organization. [J Physiol Sci. 2008;58 Suppl:S68]
