EPITHELIUM-DEPENDENT RESPONSES OF SEROTONIN IN A COAXIAL BIOASSAY SYSTEM

Serotonin (10(-6)-10(-4) M) produced relaxations in a concentration-de pendent (at 10(-6) and 10(-5) M concentrations) manner followed by a c ontraction (at 10(-4) M concentration) in a co-axial system, which con sisted of guinea-pig trachea as a donor organ for epithelial derived-r elaxing factor(s) and phenylephrine-precontracted rat anococcygeus mus cle as assay tissue. Serotonin produced a concentration-dependent cont raction only in precontracted rat anococcygeus muscle mounted alone or mounted co-axially within epithelium-denuded trachea. Indomethacin (1 0(-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(-6) and 10(-5) M concentrations of serotonin), but did not affect the con traction. Imipramine (10(-8) M) and hydrocortisone (3 X 10(-5) M) redu ced the initial relaxations (from 20.5 +/- 1.6 to 3.8 +/- 1.5% and fro m 32.1 +/- 6.4 to 18.9 +/- 3.9% at 10(-6) M and 10(-5) M concentration s of serotonin, respectively) and also converted the serotonin (10(-4) M)-induced contraction to a relaxation. In the co-axial system with t rachea from guinea-pigs previously sensitized with i.p. injected egg-o valbumin, the serotonin-induced biphasic response was converted to a c ontractile response only after ovalbumin challenge. Histopathologic ch anges were observed in the epithelium of challenged tracheas taken fro m sensitized guinea-pigs and alterations of serotonin-induced epitheli um-dependent responses were attributed to the morphological and/or fun ctional damage of tracheal epithelium caused by ovalbumin challenge. I n the modified co-axial system, phenylephrine-induced contractions fad ed quickly when the rat anococcygeus muscle was mounted in epithelium- intact guinea-pig trachea, and the percentage fade was significantly h igher (92.3 +/- 2.8%) than that obtained when the anococcygeus muscle was mounted in epithelium-denuded trachea (56.9 +/- 8.4%) or when it w as 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.