Abstract
1. Effects of adenosine and the adenosine nucleotides (ATP, ADP, AMP and CoA) on the guinea-pig taenia caecum were studied by measuring simultaneously the changes in membrane potential and in contractility induced by these agents using the sucrose-gap technique.
2. In the presence of carbachol (5 × 10-8 M) the hyperpolarization and the relaxation induced by these agents was found to be closely correlated. Their different potencies suggested an action on the P2 type purinoceptor: ATP ≥ ADP ≥ AMP > CoA > AD.
3. The transmitter released endogenously in the presence of carbachol (5 × 10-8 M) by the non-adrenergic inhibitory (n.a.i.) nerves after half-maximal stimulation induced an inhibitory junction potential (i.j.p.) which transiently counteracted the carbachol-induced depolarization. This i.j.p. was mimicked by ATP, ADP, and AMP applied exogenously in concentrations of about 3 × 10-5 M, by CoA (3 × 10-4 M) and by adenosine (3 × 10-3 M).
4. The results presented are in agreement with the hypothesis that the transmitter substance released by the n.a.i. nerves is a purine nucleotide, which in the guinea-pig taenia caecum affects the smooth muscle cell membrane via the P2-purinoceptor. Of the putative transmitters studied, ATP, ADP and AMP seem the most likely.
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Selected References
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