A comparison of effects evoked in guinea-pig taenia caecum by purine nucleotides and by `purinergic' nerve stimulation

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 P 2 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 P 2 -purinoceptor. Of the putative transmitters studied, ATP, ADP and AMP seem the most likely.