Evidence for two distinct cross-bridge populations in tetanized frog muscle fibers stretched with moderate velocities.

ABSTRACT

When a tetanized frog skeletal muscle fiber is stretched with moderate velocities (< 1 L0/S), the tension developed above the level of isometric tension starts to decay after a sudden reduction of stretch velocity by more than 40-50%, though the fiber is still being stretched. We analyzed the decay of tension responses caused by the sudden reduction of stretch velocity, by applying three different types of stretch, i.e. a 1.5% stretch with velocity V1 (stretch 1), a 1.5% stretch with velocity V2 < V1 (stretch 2), and a 3% stretch consisting of stretches 1 and 2 applied in succession (stretch 3) and comparing the corresponding tension responses, TR 1, TR 2 and TR 3. It was found that TR 3 to stretch 3 was equal to algebraical sum of TR 1 to the preceding stretch 1 and TR 2 to the subsequent stretch 2. In other words, TR 2 started on the falling tension baseline equal to the decay of TR 1 after completion of stretch 1, These results can be explained by assuming two distinct cross-bridge populations mechanically acting in parallel with each other.