A comparison of effects evoked in guinea‐pig taenia caecum by purine nucleotides and by ‘purinergic’ nerve stimulation

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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.