In in vitro experiments adenosine 5′‐triphosphate (ATP) and adenosine were found to exert different effects on the guinea‐pig isolated trachea depending on whether the trachea had previously been contracted with acetylcholine (ACh) (6.6 × 10−6 m) or was at resting tone. ATP and adenosine (10−5 and 10−3 m) were equipotent in relaxing the precontracted guinea‐pig trachea, since concentrations of 1.09 ± 0.35 and 0.39 ± 0.16 mm respectively reduced by 25% the ACh‐induced contraction. ATP and adenosine (10−5 and 10−4 m) caused a moderate contraction of the guinea‐pig trachea under resting tone. This effect was antagonized by inhibitors of cyclo‐oxygenase (indomethacin 10−6m, aspirin 0.3 × 10−3m and 3 × 10−3m) and of thromboxane synthetase (nictindole 10−7m, imidazole 5 × 10−5 m), which suggests an indirect mechanism of action with release of arachidonic acid derivatives.

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373ChemInform Abstract (69 refs.).

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