Myosin light chain phosphorylation is associated with a decrease in the energy cost for contraction in fast twitch mouse muscle.

The rate of splitting of energy-rich phosphate compounds and the extent of myosin light chain phosphorylation in contracting mouse extensor digitorum longus (EDL, fast twitch) and soleus (slow twitch) muscles were studied at 20 degrees C. The rate of high energy phosphate-splitting during a maintained isometric tetanus was 1.44 +/- 0.21 mumol . g-1 . s-1 in soleus. In EDL, the splitting rate was higher, 3.71 +/- 0.62 mumol . g-1 . s-1, during the first several seconds and thereafter was reduced to a rate of 1.63 +/- 0.35 mumol . g-1 . s-1 between 12 and 15 s of stimulation. Light chains identified on 2-dimensional gel electrophoretograms from EDL corresponded to the light chain composition of fast twitch muscle (LC1f, LC2f, and LC3f). Soleus is composed of fast twitch and slow twitch fibers because 2 additional light chains were found: LC1s and LC2s. In unstimulated EDL and soleus muscles, 0.1 of the LC1f and LC2s were phosphorylated. Upon stimulation, only LC2f, and only in EDL, increased its extent of phosphorylation. The time course of increase in phosphorylation of LC2f and decrease in rate of high energy phosphate-splitting correspond so that the 2 processes may be mechanistically related. If so, it appears that myosin LC2f phosphorylation represents a thick filament regulatory system capable of downward modulation of actomyosin ATPase in vivo during a maintained contraction.