Epithelium-dependent responses of serotonin in a co-axial bioassay system

Abstract

Serotonin (10 −6–10 −4 M) produced relaxations in a concentration-dependent (at 10 −6 and 10 −5 M concentrations) manner followed by a contraction (at 10 −4 M concentration) in a co-axial systems, which consisted of guinea-pig trachea as a donor organ for epithelial derived-relaxing factor(s) and phenylephrine-precontracted rat anococcygeus muscle as assay tissue. Serotonin produced a concentration-dependent contraction only in precontracted rat anococcygeus muscle mounted alone or mounted co-axially within epithelium-denuded trachea. Indomethacin (10 −6 M) significantly inhibited the initial relaxations (from 25.1 ± 7.8 to 7.8 ± 5.0% and from 35.6 ± 8.7 to 10.4 ± 8.3% at 10 −6and 10 −5M concentrations of serotonin), but did not affect the contraction. Imipramine (10 −8 M) and hydrocortisone (3 × 10 −5 M) reduced the initial relaxations (from 20.5 ± 1.6 to 3.8 ± 1.5% and from 32.1 ± 6.4 to 18.9 ± 3.9% at 10 −6M and 10 −5M concentrations of serotonin, respectively) and also converted the serotonin (10 −4 M)-induced contraction to a relaxation. In the co-axial system with trachea from guinea-pigs previously sensitized with i.p. injected egg-ovalbumin, the serotonin-induced biphasic response was converted to a contractile response only after ovalbumin challenge. Histopathologic changes were observed in the epithelium of challenged tracheas taken from sensitized guinea-pigs and alterations of serotonin-induced epithelium-dependent responses were attributed to the morphological and/or functional damage of tracheal epithelium caused by ovalbumin challenge. In the modified co-axial system, phenylephrine-induced contractions faded quickly when the rat anococcygeus muscle was mounted in epithelium-intact guinea-pig trachea, and the percentage fade was significantly higher (92.3 ± 2.8%) than that obtained when the anococcygeus muscle was mounted in epithelium-denuded trachea (56.9 ± 8.4%) or when it was mounted alone (44.6 ± 7.7%). Our results suggest that guinea-pig tracheal epithelium is capable of modulating the responsiveness of rat anococcygeus muscle to serotonin by affecting the basal or stimulated release of some inhibitory mediators.