Inhibitory effects of catecholamines on cholinergically and non-cholinergically mediated contractions of guinea-pig isolated bronchial muscle

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Abstract
The actions of catecholamines on the responses evoked by electrical field stimulation or by acetylcholine and substance P in guinea-pig bronchial strip chain have been examined. Electrical field stimulation evoked a biphasic contraction, consisting of a cholinergically-mediated fast contraction followed by a non-cholinergically-mediated slow contraction. All catecholamines tested caused a concentration-dependent reduction in the height of the biphasic contraction, where non-cholinergic contractions were more potently inhibited. The inhibitory effect of isoprenaline was largely prevented by propranolol (2 μM) alone, whereas those of noradrenaline and adrenaline were prevented by treatment with both propranolol (2 μM) and yohimbine (2 μM). The inhibitory effect of dopamine was unaffected either by propranolol (2 μM), yohimbine (2 μM) or haloperidol (10 μM). Submaximal contractions of bronchial muscle evoked by exogenous acetylcholine (2 μM) or substance P (0.2 μM) were also inhibited by catecholamines, except dopamine, but the effects were antagonized by propranolol (2 μM) alone. The results suggest that in guinea-pig isolated bronchial muscle, catecholamines can inhibit both cholinergic and non-cholinergic excitatory neurotransmissions not only by postjunctional β-adrenoceptors but also by prejunctional α2-adrenoceptors.