EVIDENCE FOR 2 DISTINCT CROSS-BRIDGE POPULATIONS IN TETANIZED FROG-MUSCLE FIBERS STRETCHED WITH MODERATE VELOCITIES

When tetanized frog skeletal muscle fibers are subjected to moderate-v elocity stretches (<1L(0)/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 fibers are still being stret ched. We analysed the decay of tension response caused by the sudden r eduction of stretch velocity, by applying three different types of str etch to a tetanized fiber, i.e., a 1.5% stretch with velocity V-1 (str etch-1), a 1.5% stretch with velocity V-2 < V-1 (stretch-2), and a 3% stretch consisting of stretch-1 and stretch-2 applied in succession (s tretch-3) and comparing the corresponding tension responses, TR-1, TR- 2, and TR-3. It was found that TR-3 to stretch-3 resulted from algebra ical summation of TR-1 to the preceding stretch-1 and TR-2 to the subs equent stretch-2. These results can be accounted for by assuming two d istinct cross-bridge populations in stretched fibers. (C) 1998 Academi c Press.