Endogenous AA metabolites and their possible role in tracheal smooth muscle tone in guinea pigs

The effects of endogenous arachidonic acid (AA) metabolites on inherent tone and histamine-induced constriction were studied in guinea pig tracheal smooth muscle. Inhibitors of either cyclooxygenase (indomethacin) or lipoxygenase (AA 861) significantly diminished the inherent tone of the muscle. Antagonists of prostaglandins (SC 19220) or leukotrienes (FPL 55712) also diminished the inherent tone, whereas an inhibitor of thromboxane synthase (OKY 046) had no significant effect. These results show that the metabolites of the lipoxygenase pathway as well as prostaglandins also participate in the maintenance of inherent tone. To reexamine the previously reported augmentation of histamine constriction induced by the inhibitors and the antagonists, we compared the active tension of the muscle measured from the maximum relaxed level as the base line to eliminate the fluctuation of inherent tone. Such comparison revealed that the inhibitors and the antagonists have no augmentative effect on either the maximum response to histamine or the concentration required to produce 50% of maximum active tension and that there is functional synergism between the exogenously added histamine and the endogenously produced AA metabolites. Therefore the zero active tension is useful as a base line to compare the contractile response of a drug-treated preparation with that of a nontreated preparation.