Correlated reduction of velocity of shortening and the rate of energy utilization in mouse fast-twitch muscle during a continuous tetanus

Isometric tetani of slow-twitch soleus and fast-twitch extensor digitorum longus (EDL) muscles of the mouse were studied at 20 degrees C. The total energy cost for 3- and 9-s isometric tetani was measured as a function of length above L0 and partitioned into a filament overlap-dependent fraction and a smaller filament overlap-independent fraction. In both muscles, the rate of filament overlap-independent energy cost did not change with tetanic duration. In the EDL, but not in the soleus, the rate of filament overlap-dependent energy utilization was greater in a 3-s tetanus than in a 9-s tetanus. The force-velocity relationships were studied after 3 and 9 s of isometric tetanus. In the soleus, Vmax was 2 fiber lengths/s and was not dependent on the duration of isometric tetanus. In contrast, in the EDL, Vmas decreased from 5.9 fiber lengths/s at 3 s to 3.9 fiber lengths/s at 9 s. The velocity of unloaded shortening (Vus) was examined by the slack test method as a function of the duration of isometric tetanus duration over the range of 1-15 s. In the soleus, Vus did not change, whereas in the EDL, Vus declined progressively from 6.4 to 3.2 fiber lengths/s after an isometric tetanus of increasing duration from 1 to 15 s. These results cannot exclude the hypothesis that in a maintained tetanus there is a decrease in the intrinsic cross- bridge turnover rate in the fast-twitch EDL, but not in the slow-twitch soleus muscle.