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Summary1. The responses to non-adrenergic, non-cholinergic nerve stimulation have been compared with those to exogenously applied ATP on seventeen different tissues from a number of vertebrate classes.2. Stimulation of all the mammalian gut preparations studied (with the exception of the guinea-pig ileum) after blockade of the effects of adrenergic and cholinergic nerve stimulation by guanethidine (3 5 /UM) and hyoscine (1-3 /iM) caused inhibition; exogenously applied ATP mimicked this inhibitory response. 3. Stimulation of the guinea-pig ileum in the presence of hyoscine and guanethidine, usually caused a diphasic response, relaxation followed by contraction; exogenously applied ATP mimicked this response, in contrast to acetylcholine and noradrenaline which caused excitation and relaxation respectively. 4. Stimulation of preparations of lower vertebrate gut and guinea-pig bladder in the presence of hyoscine and guanethidine caused contraction; exogenously applied ATP mimicked this contractile response. 5. In each preparation the time course of the response to ATP was similar or identical to the response to non-adrenergic, non-cholinergic nerve stimu!ation. 6. The results are consistent with the hypothesis that a purine nucleotide may be the transmitter substance released from non-adrenergic, non-cholinergic nerves supplying smooth muscle preparations from a number of vertebrate classes.

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In guinea-pig trachea adenosine 5'-triphosphate (ATP), adenosine 5'-diphosphate (ADP), adenosine 5'-phosphate (AMP), adenosine and adenine were similarly potent in causing relaxation of the smooth muscle. This is in contrast to gut where ATP and ADP are 30 times more potent than adenosine. Studies with dipyridamole suggest that in trachea, as in gut, nucleotides are rapidly metabolized to adenosine. A polyphosphate modified analogue of ATP, the a,fl-methylene isostere, which resists degradation to adenosine was inactive in trachea although it is a potent relaxant in gut. This result may suggest that the intact ATP molecule is also inactive in the tracheal preparation: i.e. ATP acts only via its adenosine metabolite implying that receptors for adenosine but not ATP are present in the tissue.