Summary1. The possibility that a purine nucleotide is involved in excitatory transmission to the urinary bladder has been tested. All the purine compounds tested which contained a pyrophosphate bond produced contraction, adenosine triphosphate (ATP) being the most potent. Adenosine and adenosine monophosphate caused relaxation. 2. The response to ATP closely mimicked the nerve-mediated contraction, both being characterized by a rapid contraction which was not maintained. A lack of sensitivity to ATP was noted in some preparations of the rat urinary bladder. 3. Both nerve-mediated contractions and contractions caused by ATP were blocked by quinidine, while the response to acetylcholine persisted. 4. Nerve-mediated responses were depressed during tachyphylaxis produced by high concentrations of ATP. Tachyphylaxis did not occur when low concentrations were used. Possible explanations for these results are discussed. 5. The results are consistent with the hypothesis that non-cholinergic excitatory nerves to the guinea-pig bladder release a purine nucleotide, but do not provide critical evidence for it. IntroductionThe urinary bladder of all vertebrates studied receives an excitatory innervation

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1 Di erences in the mechanism of non-adrenergic, non-cholinergic (NANC) inhibitory responses to preganglionic-and post-ganglionic nerve stimulation were investigated in the guinea-pig isolated trachea. 2 Stimulation of the vagus nerve at frequencies above 4 Hz elicited NANC relaxation of the trachealis muscle. Responses to low frequencies of stimulation (4 ± 8 Hz) were abolished by the nitric oxide (NO) synthase inhibitor L-NOARG (10 mM), while a L-NOARG resistant component was observed at higher stimulus frequencies. The L-NOARG-resistant component of NANC inhibitory responses to higher frequencies of vagus nerve stimulation were signi®cantly attenuated by the proteinase a-chymotrypsin (2 U/ml), suggesting that a neuropeptide such as VIP may contribute to NANC responses. 3 When postganglionic nerves were stimulated by electrical ®eld stimulation (EFS), responses were readily elicited at frequencies below 4 Hz. Like responses to vagus nerve stimulation, responses to low frequency (54 Hz) EFS were abolished by L-NOARG while a L-NOARG-resistant component was apparent at higher stimulus frequencies. 4 The L-NOARG-resistant component of NANC inhibitory responses to EFS was sensitive to achymotrypsin only if stimuli were delivered in either long trains at a low frequency (4 Hz for 10 ± 30 s) or short trains of high frequency (16 Hz for 2.5 ± 7.5 s). 5 Responses to preganglionic nerve stimulation were approximately 35% of the amplitude of responses to EFS in the same preparations. 6 In conclusion, responses to preganglionic and postganglionic NANC inhibitory nerve stimulation in the guinea-pig trachea di er in maximum amplitude, frequency-response characteristics and the contributions of cotransmitters. We suggest that these di erences may be explained by ®ltering of preganglionic input to postganglionic NANC neurons. These results have implications in all studies where EFS is considered to be representative of physiological stimulation of post-ganglionic nerve stimulation.

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