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Ionic interactions in acetylcholine contraction of the denervated rat diaphragm
Abstract:
1. The nature of the drug-receptor interaction in the acetylcholine-induced contraction of the denervated rat diaphragm was studied both by altering the external ionic environment and by determining its drug sensitivity.2. The response to acetylcholine was insensitive to tetrodotoxin or saxitoxin, but was abolished by procaine.3. It was unaffected by levels of MnCl(2) sufficient to block the response of the innervated diaphragm to electrical stimulation, although higher levels reduced the response. The effect of Mn(++) on the innervated diaphragm was overcome by raising the external Ca(++) level; this was ineffective in the denervated preparation.4. In spite of its insensitivity to tetrodotoxin the acetylcholine contraction was reduced and prolonged by low external Na(+) levels. This prolongation was not found when Li(+) substituted for Na(+).5. Increasing the external level of Ca(++) or Mg(++) 3 to 5-fold reduced the acetylcholine contraction; high Ca(++) also prolonged it. Reduction in the divalent cation level was without effect.6. Procaine inhibition of the acetylcholine response was largely competitive, as was inhibition due to (+)-tubocurarine. This was shown by probit analysis and the dose-ratio test.7. Thiocyanate (12 mM) augmented and prolonged the contraction; this action was modified by altering the Ca(++) or Mg(++) level of the solution.8. The acetylcholine receptor resembles that of the innervated postsynaptic membrane.
Insights
The acetylcholine receptor in denervated rat diaphragm muscle responds differently to ions than expected. Its contraction is sensitive to sodium levels and certain drugs, suggesting a unique interaction mechanism.
Area of Science:
- Pharmacology
- Neuroscience
- Muscle Physiology
Background:
- The acetylcholine receptor's function is crucial for muscle contraction.
- Understanding drug-receptor interactions aids in developing targeted therapies.
- Denervated muscle presents a unique model for studying receptor behavior.
Purpose of the Study:
- To investigate the drug-receptor interaction in acetylcholine-induced contractions of denervated rat diaphragm.
- To determine the sensitivity of this interaction to various ionic environments and drugs.
Main Methods:
- Altering external ionic concentrations (Na+, Ca++, Mg++, Mn++).
- Testing the effects of specific blockers like tetrodotoxin, saxitoxin, and procaine.
- Evaluating drug sensitivity using probit analysis and dose-ratio tests for competitive inhibition.
Main Results:
- Acetylcholine response was insensitive to tetrodotoxin/saxitoxin but blocked by procaine.
- Low Na+ reduced and prolonged contraction; Li+ substitution did not cause prolongation.
- Increased Ca++ or Mg++ reduced contraction, while high Ca++ also prolonged it.
- Procaine and (+)-tubocurarine showed largely competitive inhibition.
- Thiocyanate augmented and prolonged contraction, modulated by divalent cations.
Conclusions:
- The acetylcholine receptor in the denervated rat diaphragm exhibits distinct ionic and drug sensitivities.
- The receptor's characteristics suggest similarities to the innervated postsynaptic membrane.
- Ionic environment significantly modulates acetylcholine receptor-mediated muscle contraction.