Increased force development rates of fatigued mouse skeletal muscle are graded to myosin light chain phosphate content.

Phosphorylation of myosin regulatory light chain (R-LC) increases the Ca2+ sensitivity of cross-bridge transitions, which determine rate of force development in skinned skeletal muscle fibers. The purpose of this study was to determine whether phosphorylation of R-LC is the molecular basis for the increased force development rates (+dF/dtmax) observed in fatigued mouse extensor digitorum longus muscle (EDL) (stimulated in vitro at 25 degrees C). Parameters of twitch and tetanic force were obtained after the application of different-frequency conditioning stimuli (CS), which were used to vary R-LC phosphorylation and reduce peak tetanic force (Po). Without CS, R-LC phosphorylation (in moles phosphate per mole R-LC) was not elevated above rest (0.11 +/- 0.02 vs. 0.13 +/- 0.02, respectively), and no aspect of the twitch (Pt) Po was altered. Stimulating muscles at 2.5-20 Hz increased R-LC phosphorylation in a frequency-dependent manner, from 0.23 +/- 0.04 to 0.82 +/- 0.03, respectively. Moreover, stimulation at 2.5-20 Hz potentiated Pt (range: 4 +/- 2-28 +/- 2%), increased the +dF/dtmax of potentiated twitches (range: 5 +/- 1-28 +/- 2%), and reduced Po (range: 6 +/- 1-21 +/- 1%). Higher-frequency stimulation (40 or 100 Hz) did not phosphorylate R-LC or potentiate Pt or twitch +dF/dtmax further. Stimulation at 40 and 100 Hz did, however, have different effects on Po compared with 20-Hz data (Po reduced 27 +/- 2 and 11 +/- 2%, respectively). The increased +dF/dtmax of potentiated twitches observed after different CS procedures were graded to R-LC phosphorylation (r = 0.97, P < 0.001). It is concluded that phosphorylation of R-LC increases extent of twitch force development in mouse EDL muscle fatigued by CS.