Comparative study on different responses of vascular and extravascular smooth muscles mounted inside the guinea-pig...

The difference between the responses of phenylephrine (1 microM)-precontracted vascular (endothelium-denuded rat or rabbit aortic strips) and nonvascular (rat anococcygeus muscle) smooth muscles to acetylcholine (0.1-100 microM) was investigated when they were mounted co-axially inside the tracheas isolated from normal or ovalbumin-sensitized guinea-pigs. Acetylcholine produced concentration-dependent relaxations in both types of bioassay tissues. These relaxations, previously shown to be due to the release of airway epithelium-derived relaxing factor(s), were significantly attenuated when the epithelial layer of the tracheas was removed mechanically (as confirmed by histological examination). There were no significant differences in responsiveness to acetylcholine between vascular strips mounted inside the epithelium-intact normal or sensitized tracheas. The phenylephrine-induced precontraction was significantly more pronounced in rat anococcygeus muscles mounted inside sensitized tracheas as compared to tissues mounted inside control tracheas. The acetylcholine-induced relaxations were significantly decreased but this effect disappeared when the concentration of phenylephrine was reduced to obtain a similar precontraction level as in tissues mounted inside control tracheas. The responsiveness of both vascular strips and anococcygeus muscles to acetylcholine was attenuated when they were mounted inside sensitized tracheas and incubated with ovalbumin for 20 min, which may be explained by the epithelial damage induced by ovalbumin challenge. This attenuation was absent when co-axial pairs, utilizing normal tracheas, were used. These results indicate a difference in response patterns of the rat anococcygeus muscle and vascular strips in ovalbumin-sensitized tracheas, which should be taken into consideration in co-axial bioassay studies.