Effect of disuse on sarcoplasmic reticulum in fast and slow skeletal muscle.

The effect of 6 wk of hindlimb immobilization on rat skeletal muscle sarcoplasmic reticulum (SR) was determined in the slow-twitch, type I soleus (SOL), the fast-twitch, type IIA deep region of the vastus lateralis (DVL), and the fast-twitch, type IIB superficial region of the vastus lateralis (SVL). Immobilization produced a significant decline in the Ca2+ uptake rate (Vmax) of SR vesicles from the slow SOL (0.930 +/- 0.116 to 0.365 +/- 0.071 mumol Ca2+ . mg-1 . min-1), while the SR Vmax increased in the fast SVL (2.763 +/- 0.133 to 5.209 +/- 0.687) and was unaltered in the DVL. Vesicles from the fast SVL and DVL also exhibited a higher total Ca2+ uptake capacity following immobilization. An evaluation of the time course of the immobilization-mediated effect revealed an increased Ca2+ uptake capacity in all three samples after 1 wk. In the SOL total Ca2+ uptake returned to control level after 2 wk, while in the fast-twitch muscles the higher capacities were maintained. The Ca2+-stimulated SR ATPase activity was not altered in any of the muscles studies, although the total SR ATPase activity increased twofold in the slow SOL.