Potentiation of in vitro concentric work in mouse fast muscle

Phosphorylation of myosin regulatory light chain (R-LC) is associated with potentiated work and power during twitch afterloaded contractions in mouse extensor digitorum longus muscle [R. W. Grange, C. R. Cory, R. Vandenboom, and M. E. Houston. Am. J. Physiol. 269 (Cell Physiol. 38): C713-C724, 1995]. We now describe the association between R-LC phosphorylation and potentiated concentric work when the extensor digitorum longus muscle is rhythmically shortened and lengthened to simulate contractions in vivo. Work output (at 25 degrees C) was characterized at sine frequencies of 3, 5, 7, 10, and 15 Hz at excursions of 0.6, 1.2, and 1.6 mm (approximately 5, 9, and 13% optimal muscle length) at a low level of R-LC phosphorylation. Muscles stimulated during the sine function with a single twitch at specific times before or after the longest muscle length yielded maximal concentric work near the longest muscle length at a sine frequency of 7 Hz (e.g., excursion approximately 9% optimal muscle length = 1.6 J/kg). Power increased linearly between sine frequencies of 3 and 15 Hz at all excursions (maximum approximately 29 W). After a 5-Hz 20-s conditioning stimulus and coincident with a 3.7-fold increase in R-LC phosphate content (e.g., from 0.19 to 0.70 mol phosphate/mol R-LC), work at the three excursions and a sine frequency of 7 Hz was potentiated a mean of 25, 44, and 50% (P < 0.05), respectively. The potentiated work during rhythmic contractions is consistent with enhanced interaction between actin and myosin in the force-generating states. On the basis of observations in skinned skeletal muscle fibers (H. L. Sweeney and J. T. Stull. Proc. Natl. Acad. Sci. USA 87:414-418, 1990), this enhancement could result from increased phosphate incorporation by the myosin R-LC. Under the assumption that the predominant effect of the conditioning stimulus was to increase R-LC phosphate content, our data suggest that a similar mechanism may be evident in intact muscle.