Variations in the Relative Potencies of Bronchodilators in the Presence and Absence of Carbachol-Induced Tone

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Abstract
Responses to dl-isoprenaline, aminophylline, and the bacteriostatic preservative methyl paraben were obtained on the isolated guinea pig trachea in the absence and presence of the spasmogen carbachol. The results demonstrate that the presence of carbachol significantly reduced the relaxant activity of dl-isoprenaline (one-quarter as potent) whereas the activity of aminophylline and methyl paraben were only reduced by one-half. It is concluded that the use of a "pharmacological tool" such as carbachol can alter the interpretation of results obtained in such studies since the beta sympathomimetic dl-isoprenaline appears to interact with carbachol in a manner different from that of the non-beta adrenoreceptor agonists aminophylline and methyl paraben.

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