ABSENCE OF MECHANICAL EVIDENCE FOR ATTACHED WEAKLY BINDING CROSS-BRIDGES IN FROG RELAXED MUSCLE-FIBERS

1. Passive force responses to ramp stretches at various velocities wer e measured in intact and skinned single muscle fibres isolated from th e lumbricalis muscle of the frog. Force was measured using a fast capa citance transducer and sarcomere length was measured using a laser lig ht diffraction technique at a point very close to the fixed end so as to avoid effects of fibre inertia. Experiments were performed at 15 de grees C with sarcomere length between 2.13 and 3.27 mu m under high (1 70 mM) and low (20 mM) ionic strength. 2. The analysis shows that the force response is the sum of at least three components: (i) elastic (f orce proportional to the amount of stretch), (ii) viscous (force propo rtional to rate of stretch), and (iii) viscoelastic (resembling the re sponse of a pure viscous element in series with an elastic element). 3 . The amplitude of all these components increased progressively with s arcomere length in the whale range measured. 4. A further component, a ttributable to the short-range elasticity (SREC), was present in the f orce response of the intact fibres. 5. The amplitude of the force resp onse decreased substantially upon skinning at high ionic strength but increased again at low ionic strength. The SREC was completely abolish ed by skinning. 6. None of the components of the force response was fo und to have the properties expected from the previously postulated 'we akly binding bridges'. OC with sarcomere length between 2.13 and 3.27 mu m under high (170 mM) and low (20 mM) ionic strength. 2. The analys is shows that the force response is the sum of at least three componen ts: (i) elastic (force proportional to the amount of stretch), (ii) vi scous (force proportional to rate of stretch), and (iii) viscoelastic (resembling the response of a pure viscous element in series with an e lastic element). 3. The amplitude of all these components increased pr ogressively with sarcomere length in the whale range measured. 4. A fu rther component, attributable to the short-range elasticity (SREC), wa s present in the force response of the intact fibres. 5. The amplitude of the force response decreased substantially upon skinning at high i onic strength but increased again at low ionic strength. The SREC was completely abolished by skinning. 6. None of the components of the for ce response was found to have the properties expected from the previou sly postulated 'weakly binding bridges'.