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Threshold for force potentiation associated with skeletal myosin phosphorylation
R Vandenboom1, R W Grange, M E Houston
1Department of Kinesiology, University of Waterloo, Ontario, Canada.
The American Journal of Physiology
|December 1, 1993
Summary
Myosin light chain phosphorylation enhances muscle force at low stimulation frequencies but not high ones. This suggests a frequency limit for phosphorylation-mediated force potentiation in fast-twitch muscles.
Area of Science:
- Muscle Physiology
- Biochemistry
Background:
- Myosin light chain (P-LC) phosphorylation is linked to isometric twitch force potentiation in fast-twitch muscles.
- Understanding the relationship between P-LC phosphorylation and force potentiation at various stimulation frequencies is crucial.
Purpose of the Study:
- To investigate the association between myosin P-LC phosphorylation and force potentiation in mouse extensor digitorum longus (EDL) muscles.
- To determine the effects of higher stimulation frequencies (1-150 Hz) on this relationship.
Main Methods:
- Mouse EDL muscles were stimulated at 25°C.
- Isometric force and rate of force development (+dF/dtmax) were measured before and after a conditioning stimulus (5 Hz, 20 s) that increases P-LC phosphorylation.
- Measurements were taken at various test frequencies (1-150 Hz).
Main Results:
- Myosin P-LC phosphorylation increased ninefold, leading to a 20-37% increase in +dF/dtmax across all tested frequencies.
- Peak isometric force potentiation (approx. 15%) was observed only at stimulation frequencies of 1-15 Hz.
- At frequencies of 20-150 Hz, peak force decreased by 5-40% despite increased +dF/dtmax.
Conclusions:
- The stimulation frequency limit for force potentiation mediated by myosin P-LC phosphorylation in mouse EDL muscle at 25°C is below 20 Hz.
- Increased cross-bridge transition rates due to P-LC phosphorylation may explain the enhanced +dF/dtmax and low-frequency force potentiation.