Analysis of the length response to a force step in smooth muscle from rabbit urinary bladder

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Abstract
Responses to isotonic quick release of AC‐stimulated smooth muscle strips from rabbit urinary bladder were analysed. Releases were performed at the peak of contraction and at a preset tension level in the contraction and relaxation phase. In other expts. responses at 37°C and 27°C were compared. The length response always consisted of 3 parts: (1) elastic recoil, (2) rapid length change (isotonic transient), (3) steady length change. Qualitatively, phases (1)‐(3) could be distinguished also in responses to isotonic quick stretch. The immediate elastic recoils, phase (1), were described by exponential stress‐strain relations. Stiffness was found to be somewhat lower during relaxation than during contraction. No effect of temperature on the elastic recoil was seen. The initial velocity in phase (2) was 2–3 times greater than the velocity 100 ms after release. By means of computer analysis of the length records during phases (2) and (3) two decaying exponential processes with widely different time constants could be separated. The time constant of the faster process was of the order of 15–30 ms at 37°C. It decreased with increasing force steps and with increasing temperature. The amount of shortening associated with this process was correlated with the size of the force step, reaching a maximum of about 1.2% of the muscle length. The shortening velocities in phase (3), measured 100 ms after release, were described by Hill's equation. Vmax in the rising part and at the peak of contraction were 0.7 and 0.6 L/s respectively at 37°C. Lower values were found during relaxation and at 27°C. We suggest that part of the elastic recoil in phase (1) occurs in structures associated with the individual cross‐bridges, that phase (2) is dominated by a change in the distribution of conformations of bridges in the attached position and that the shortening rate in phase (3) is determined by the entire cycle of events during turnover of bridges after the muscle has adapted to the new load. Observations on the force response to length steps and on shifts from isometric to afterloaded isotonic contraction and vice versa are consistent with this interpretation.