Influence of the epithelium on responsiveness of guinea‐pig isolated trachea to contractile and relaxant agonists

1 The potency (pD2) and maximal contractile effect (Emax) of histamine, acetylcholine, carbachol and K+ were assessed from cumulative concentration‐effect curves in guinea‐pig isolated tracheal ring preparations with and without an intact epithelium. 2 Estimates of Emax were not significantly different in epithelium‐denuded preparations compared with those measured in intact preparations; pD2 values for acetylcholine, carbachol and K+ were not significantly altered. In contrast, the potency of histamine was significantly increased by about 4 fold in preparations devoid of epithelial cells. 3 Estimates of potency and Emax were also determined for the smooth muscle relaxants isoprenaline, forskolin and theophylline (which increase intracellular cyclic AMP) and for nitroglycerin (which increases cyclic GMP) in both intact and epithelium‐stripped tracheal rings. The pD2 values for these relaxants were not significantly altered by the removal of the epithelium. However, with the exception of nitroglycerin, Emax values for these relaxants were significantly lower in stripped than in intact tracheal rings that had been maximally precontracted with carbachol. 4 The autoradiographic localisation of binding sites for the non‐selective β‐adrenoceptor ligand [125I]‐iodocyanopindolol (I‐CYP) showed that the epithelium of the guinea‐pig trachea had a 75 ± 16% greater density of β‐adrenoceptors than the smooth muscle. Removing the epithelium did not significantly alter either the density of smooth muscle binding sites or the affinity of I‐CYP binding. It was concluded that the reduced functional response of guinea‐pig trachea to isoprenaline was probably not due to smooth muscle β‐adrenoceptor dysfunction. 5 Results indicate that the epithelium plays an important role in the modulation of responsiveness of guinea‐pig trachea to histamine and relaxants that mediate their effects by selectively increasing intracellular cyclic AMP levels.