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Force and membrane potential in acetylcholine and potassium contractures of denervated mouse muscles
Abstract:
Depolarization and contracture force (P) provoked by acetylcholine (ACh) and by K ions were studied in bundles dissected from mouse soleus muscles that had been denervated for 4-7 days. Cl-free solutions were used. The muscle fibres were depolarized by solutions containing 150 mM K or 10 microM ACh to nearly zero mV resulting in maximum P (Pmax). Threshold P was produced when the membrane was depolarized to more than about -60 mV by both agents. 50% Pmax was produced by [K] causing the membrane to depolarize to -42 mV, whereas a potential more positive than -20 mV was required for 50% Pmax to be produced when ACh was used. The rate of depolarization was always higher for ACh than for K. Pretreatment by 0.05 microM ACh (about threshold for P) did not affect the P-[K] relation appreciably showing that ACh did not "stabilize" the membrane. Nearly equal P was provoked by successive applications of just suprathreshold agent concentrations when the order of application was ACh----K but not with the reverse order. Hypertonicity (by addition of 300 mM sucrose to all solutions) caused PACh to decrease and PK to increase. It was concluded that the ACh receptors are located in the surface membrane of the muscle fibres, not in the T-system membranes.
Insights
Acetylcholine (ACh) and potassium ions (K) induce muscle contracture. This study found that ACh receptors are on the muscle fiber surface membrane, not in the T-system, impacting muscle depolarization and contracture force.
Area of Science:
- Neuroscience
- Muscle Physiology
- Cell Biology
Background:
- Denervation alters muscle fiber excitability and response to stimuli.
- Acetylcholine (ACh) and potassium ions (K) are key in muscle depolarization and contraction.
Purpose of the Study:
- To investigate the location of acetylcholine receptors in denervated mouse soleus muscles.
- To compare the effects of ACh and K ions on muscle depolarization and contracture force.
Main Methods:
- Dissection of mouse soleus muscle bundles.
- Exposure to Cl-free solutions with varying concentrations of K ions and ACh.
- Measurement of depolarization thresholds and contracture force (P).
Main Results:
- Both ACh and K ions induced depolarization and contracture force (Pmax).
- ACh required a more positive membrane potential than K for 50% Pmax.
- ACh caused faster depolarization than K, and receptors were localized to the surface membrane.
Conclusions:
- Acetylcholine receptors are located in the surface membrane of muscle fibers.
- The T-system membranes are not the primary site for ACh receptors in this model.
- Differential responses to ACh and K ions provide insight into receptor localization.