Acetylcholine and histamine interact supra-additively on bronchomotor tone in guinea-pigs in vivo: A possible consequence of the relationship between airways resistance and calibre

Abstract

Histamine and acetylcholine administered simultaneously to anaesthetized guinea-pigs results in a bronchoconstriction greater than that of the sum of the individual bronchoconstrictor effects of histamine and acetylcholine administered separately, i.e., a supra-additive interaction. In contrast, a supra-additive effect of acetylcholine and histamine on the isolated tracheal strip did not occur. At a pretreatment concentration of acetylcholine of 5 μM, histamine concentration-response curves were shifted rightwards, i.e., a less than additive interaction, suggesting that, at higher concentrations, histamine and acetylcholine do not act as independent agonists on guinea-pig tracheal smooth mus..