The roles of myosin ATPase activity and myosin light chain relative content in the slowing of type IIB fibers with hindlimb unweighting in rats

We tested the hypothesis that slowing of shortening velocity generated by type IIB fibers from hindlimb-unweighted (HU) rats resulted from a reduced ATPase activity and/or a reduction in the relative content of myosin light chain 3f isoform, content (MLCA(3f)). After 2, 3, and 4 wk of HU, maximal unloaded shortening velocity (V-o) of single permeabilized semimembranosus muscle fibers was determined by the slack test. Subsequently, the myosin heavy chain and the relative content of MLC were determined by SDS-PAGE. The ratio of MLC3f to MLC2f was determined by densitometric analysis. In addition, myofibrils were prepared from permeabilized fibers (soleus and semimembranosus muscles) and assayed for resting myosin ATPase and Ca2+-activated myosin ATPase. After HU, V-o declined by 28-40% and the MLC3f/ MLC2f ratio decreased by 32 to 48%. A significant correlation between the relative amount of MLC3f and V. was found (r = 0.48, P < 0.05). Resting myosin ATPase rates were not different between myofibrils prepared from corresponding muscles of control and HU rats (P = 0.86). Ca2+-activated myosin ATPase activities also were not different between myofibrils prepared from corresponding muscles of control and HU rats (P := 0.13). These data suggest that the slowing of maximal unloaded shortening velocity in type IIB fibers with HU is. at least in part, due to a relative change in the essential light chain composition. a decrease in the relative amount of MLC3f and most likely a concomitant increase in MLC1f. However, this reduction in V-o is independent of myosin ATPase activity.