Using electrical field stimulation of epithelium-denuded intact guinea pig tracheal tube preparations, we studied the presence and role of prejunctional ₂-adrenoceptors by measuring evoked endogenous acetylcholine (ACh) and norepinephrine (NE) release directly. Analysis of ACh and NE was through two HPLC systems with electrochemical detection. Electrical field stimulation (150 mA, 0.8 ms, 16 Hz, 5 min, biphasic pulses) released 29.1 ± 2.5 pmol ACh/g tissue and 70.2 ± 6.2 pmol NE/g tissue. Preincubation for 15 min with the selective ₂-adrenoceptor agonist fenoterol (1 µM) increased both ACh and NE overflow to 178 ± 28 (P < 0.01) and 165 ± 12% (P < 0.01), respectively, of control values, increases that were abolished completely by the selective ₂-adrenoceptor antagonist ICI-118551 (1 µM). Further experiments with increasing fenoterol concentrations (0.1-100 µM) and different preincubation periods (1, 5, and 15 min) showed a strong and concentration-dependent facilitation of NE release, with maximum response levels decreasing (from nearly 5-fold to only 2.5-fold of control value) with increasing agonist contact time. In contrast, sensitivity of facilitatory ₂-adrenoceptors on cholinergic nerves to fenoterol gradually increased when the incubation period was prolonged; in addition, a bell-shaped concentration-response relationship was found at 15 min of preincubation. Fenoterol concentration-response relationships (15-min agonist preincubation) in the presence of atropine and yohimbine (1 µM each) were similar in the case of NE release, but in the case of ACh release, the bell shape was lost. The results indicate a differential capacity and response time profile of facilitatory prejunctional ₂-adrenoceptors on adrenergic and cholinergic nerve terminals in the guinea pig trachea and suggest that the receptors on adrenergic nerves are more susceptible to desensitization.

facilitatory prejunctional ₂-adrenoceptors; acetylcholine release; norepinephrine release

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