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How do skinned skeletal muscle fibers relax?
B K Hoskins1, S Lipscomb, I P Mulligan
1University Laboratory of Physiology, University of Oxford, Parks Road, Oxford, OX1 3PT, United Kingdom.
Biochemical and Biophysical Research Communications
|January 27, 1999
Summary
Rapid calcium removal from muscle fibers causes faster relaxation than cross-bridge turnover. This indicates calcium removal actively promotes cross-bridge detachment, involving phosphate and ADP release during muscle relaxation.
Area of Science:
- Muscle physiology
- Biophysics
- Skeletal muscle contraction
Background:
- Muscle relaxation is a complex process involving cross-bridge detachment.
- The role of calcium removal in accelerating cross-bridge decay is not fully understood.
Purpose of the Study:
- To investigate the kinetics of muscle relaxation following rapid calcium removal.
- To elucidate the role of cross-bridge detachment and release steps in muscle relaxation.
Main Methods:
- Skinned muscle fibers were used to rapidly remove free calcium with the photolabile chelator diazo-2.
- Muscle relaxation was monitored by changes in sarcomere length.
- The effects of increased free phosphate (Pi) and magnesium adenosine di-phosphate (MgADP) on relaxation were assessed.
Main Results:
- Rapid calcium removal induced muscle relaxation faster than cross-bridge turnover, without significant sarcomere length change.
- Muscle relaxation occurred in two phases: a truncated linear phase (Phase 1) and a rapid exponential phase (Phase 2).
- Increased Pi and MgADP levels slowed relaxation, suggesting Pi and ADP release are crucial steps.
Conclusions:
- Calcium removal not only prevents new cross-bridge formation but also actively promotes the detachment of force-generating cross-bridges.
- Muscle relaxation involves distinct Pi and ADP release steps.
- MgADP influences early relaxation phases, supporting cooperative models of cross-bridge attachment and thin filament activation.