Abstract
1. An increased uptake of labelled sodium was found in the end-plate region of rat diaphragm following brief exposure to solution containing 24Na plus carbachol (100 μM), with a wash in inactive saline. Tetrodotoxin (0·1 μM) was also present. Comparable results were obtained with decamethonium and suberyldicholine.
2. With carbachol (100 μM) the influx of labelled sodium at the end-plate region was increased by a factor of at least three as compared with that at the end of the fibres.
3. After entry the labelled sodium spread along the fibres with an apparent diffusion coefficient which was half that expected in the external solution.
4. The dose—response curve for the effect of carbachol gave a half-maximal value of 72 μM.
5. In muscles depolarized by potassium methyl sulphate the effect of carbachol on the entry of sodium was reduced although demonstrable.
6. The entry of labelled sodium at the end-plate was maintained during prolonged exposure to carbachol (100 μM) or decamethonium (100 μM).
7. The rate of entry of 24Na obtained with carbachol, after corrections for the wash, was estimated as 1·5 × 103 ions channel-1 sec-1, measured over a period of 15 sec.
8. Labelled decamethonium and labelled carbachol also accumulated at the end-plate region. After extrapolation to allow for the effects of the wash the entry of decamethonium when expressed as a clearance (pl. mg-1 sec-1) was comparable to that of sodium, as expected if decamethonium and sodium enter through the same channels.
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Selected References
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