Interaction between clonidine and histamine on the guinea-pig isolated trachea

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Abstract
In experiments on guinea-pig isolated tracheal spirals, clonidine, in concentrations of 10−6 to 3 times 10−4 M. had a contracting effect which was strongly inhibited by prazosin but not significantly modified by yohimbine. Moreover, clonidine (3 times 10−5 to 3 times 10−4 M) potentiated histamine-induced contractions; this latter effect was inhibited specifically by α1-adrenoceptor antagonists (e.g. prazosin, AR-C 239) but unmodified by yohimbine, nicardipine or agents acting on the arachidonic acid cascade. It would therefore appear that clonidine in high concentrations contracts the guinea-pig trachea by stimulating α1-adrenoceptors and that, contrary to what has been reported with other animal species, notably the dog, the guinea-pig trachea is devoid of α2-adrenoceptors that mediate contractions.