Interaction between aspirin and prostaglandins in the isolated guinea-pig tracheal muscle

The effects of PGE2 and PGF2 alpha on the tonus of isolated guinea-pig tracheal chain were investigated and compared with those of histamine and acetylcholine. PGE2 reduced tonus in normal resting state, but elevated tracheal tonus reduced by aspirin. Such PGE2-induced contractions did not exceed the initial resting tonus, and the magnitude and duration of the contractions progressively diminished with increase of PGE2 concentrations. Aspirin produced neither relaxation nor contraction in the presence of a low dose of PGE2. Unlike PGE2, PGF2 alpha produced a dose-related contraction in the normal tracheal chain, and the contractile response to PGF2 alpha was markedly potentiated by aspirin. In the presence of PGF2 alpha, aspirin no longer produced tracheal relaxation but produced a dose-related contraction. The contractile effect of histamine but not of acetylcholine was also potentiated by aspirin, but there was a slight difference between PGF2 alpha and histamine in that the potentiation of action of PGF2 alpha by aspirin was more easily diminished by PGE2. These results suggest that PGE2 plays an important role in the maintenance of the resting tonus of the isolated guinea-pig tracheal chain, and in large doses it also acts as a tracheal relaxant and attenuates the tracheal responses to PGF2 alpha and histamine.