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

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.

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 degrees C and 35 degrees C at sarcomere lengths ranging from 2.35 microm to 3.85 microm. Twitch contractions were assessed before and after 1.5 s of 75 Hz stimulation at 22 degrees C or during 10 s of 10 Hz stimulation at 22 degrees C or 35 degrees C.

Staircase potentiation was greater at 35 degrees C than 22 degrees C, and the relative magnitude of the twitch contraction (Pt*/Pt) was proportional to sarcomere length with a negative slope, over the range 2.3 microm - 3.7 microm. Linear regression yielded the following: Pt*/Pt = -0.59 x SL+3.27 (r2 = 0.74); Pt*/Pt = -0.39 x SL+2.34 (r2 = 0.48); and Pt*/Pt = -0.50 x SL+2.45 (r2 = 0.80) for staircase at 35 degrees C, and 22 degrees 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.

There is no evidence that posttetanic potentiation is fundamentally different from staircase in these muscles.