Epithelium-dependent relaxation of guinea-pig tracheal smooth muscle by carbachol.

Modulation of the effect of carbachol by epithelium on tracheal smooth muscle of guinea-pig was studied by using coaxial bioassay system. The contractile response of the bioassay organ (epithelium-free tracheal smooth muscle of guinea-pig) to carbachol was well maintained along the observation period of 30 min. When the same bioassay organ was taken into epithelium-intact tracheal tube of guinea-pig, a significant relaxation followed the carbachol-induced initial contraction within 30 min which was abolished by the removal of epithelial cell layer. Epithelium-dependent relaxation with carbachol is not due to stimulation of M1-muscarinic receptors. The results indicate that airway epithelium has a role in the termination of carbachol-induced bronchoconstriction.