Sarcomere length-dependence of activity-dependent twitch potentiation in mouse skeletal muscle

BACKGROUND: It has been reported that potentiation of a skeletal muscle twitch response is proportional to muscle length with a negative slope during staircase, and a positive slope during posttetanic potentiation. This study was done to directly compare staircase and posttetanic responses with measurement of sarcomere length to compare their length-dependence. METHODS: Mouse extensor digitorum longus (EDL) muscles were dissected to small bundles of fibers, which permit measurement of sarcomere length (SL), by laser diffraction. In vitro fixed-end contractions of EDL fiber bundles were elicited at 22°C and 35°C at sarcomere lengths ranging from 2.35 μm to 3.85 μm. Twitch contractions were assessed before and after 1.5 s of 75 Hz stimulation at 22°C or during 10 s of 10 Hz stimulation at 22°C or 35°C. RESULTS: Staircase potentiation was greater at 35°C than 22°C, and the relative magnitude of the twitch contraction (P(t)*/P(t)) was proportional to sarcomere length with a negative slope, over the range 2.3 μm – 3.7 μm. Linear regression yielded the following: P(t)*/P(t )= -0.59·SL+3.27 (r(2 )= 0.74); P(t)*/P(t )= -0.39·SL+2.34 (r(2 )= 0.48); and P(t)*/P(t )= -0.50·SL+2.45 (r(2 )= 0.80) for staircase at 35°C, and 22°C and posttetanic response respectively. Posttetanic depression rather than potentiation was present at long SL. This indicates that there may be two processes operating in these muscles to modulate the force: one that enhances and a second that depresses the force. Either or both of these processes may have a length-dependence of its mechanism. CONCLUSION: There is no evidence that posttetanic potentiation is fundamentally different from staircase in these muscles.