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 x 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 x 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 x 10(-5)M, by CoA (3 x 10(-4)M) and by adenosine (3 x 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.