Inhibitory effects of sympathomimetic drugs on cholinergically mediated contractions of guinea-pig isolated tracheal muscle

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Abstract
The experiments examine the actions of sympathomimetic drugs on the responses evoked by electrical field stimulation or by acetylcholine in guinea-pig tracheal strip chains. Electrical field stimulation evoked contractions which were cholinergically mediated, in the presence of guanethidine (10 μm) and indomethacin (2 μm). All the sympathomimetic drugs tested caused a concentration-dependent reduction in the height of these contractions. Inhibitory effects of isoprenaline and terbutaline were largely prevented by propranolol (2 μm) alone, whereas those of clonidine, oxymetazoline, lidamidine and WHR1370 were prevented by yohimbine alone (2 μm). Treatments with both propranolol and yohimbine were required to prevent the inhibitory effects of noradrenaline, adrenaline and dopamine. Contractions evoked by exogenous acetylcholine (0.1–3 μm) were also inhibited by all catecholamines and terbutaline, but not by clonidine, oxymetazoline, lidamidine and WHR1370. The inhibitory effects were antagonized by propranolol (2 μm) alone. The results suggest that in guinea-pig isolated tracheal muscle, sympathomimetic drugs can inhibit cholinergic neurotransmission not only by postjunctional β2-adrenoceptors but also by prejunctional α2-adrenoceptors.