Ionic interactions in acetylcholine contraction of the denervated rat diaphragm.

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.