Excitatory prejunctional beta 2-adrenoceptor distribution within equine airway cholinergic nerves

We examined the effect of activation of beta 2-adrenoceptor (AR) by isoproterenol (ISO) on acetylcholine (ACh) release evoked by electrical field stimulation (EFS: 20 V, 0.5 Hz, 0.5 msec) from cholinergic nerves in five regions of equine airways. We also tested if the effect of ISO was dependent on epithelium or prostanoids by examining the effect of ISO on ACh release in the presence and absence of epithelium or cyclooxygenase inhibition. Trachealis strips or bronchial rings were preincubated for 60 min with 10(-7) M atropine, 10(-6) M neostigmine, and 10(-5) M guanethidine. The ACh amount was measured by high-performance liquid chromatography with electrochemical detection. Isoproterenol (10(-8)-10(-6) M) augmented ACh release throughout the whole airway in a concentration-dependent manner. Rubbing off the epithelium potentiated EFS-induced ACh release but neither epithelium removal nor cyclooxygenase inhibition affected the magnitude of ISO-induced augmentation of ACh release. These results indicate that in equine airway parasympathetic nerves: (1) excitatory prejunctional beta 2-adrenoceptors are distributed throughout the tracheobronchial tree; (2) the function of this excitatory beta 2-adrenoceptor is independent of endogenous prostanoids and epithelium; and (3) there is an epithelium-derived factor that inhibits ACh release.