Inhibition of the excitatory non-adrenergic, non-cholinergic neurotransmission in the guinea pig tracheo-bronchial tree mediated by alpha 2-adrenoceptors.

Guinea pig main bronchi were studied in vitro for the presence of motor innervation. Field stimulation of the main bronchi revealed that besides the excitatory cholinergic component a slow and atropine resistant contraction similar to that previously found in the hilus bronchi (Grundström et al. 1981a) was present also at this level of the airways. This slow contraction was blocked by tetrodotoxin whereas it was left unaffected by a number of conventional antagonists (i.e. atropine, guanethidine, propranolol, yohimbine and prazosin) suggesting that it was due to activation of non-adrenergic, non-cholinergic neurones. In the presence of a beta-adrenoceptor blocking drug, noradrenaline inhibited the slow contractions induced by field stimulation in both the main and hilus bronchi. By contrast, contractions elicited by histamine were left unaffected by noradrenaline. In order to characterize the receptor for noradrenaline the effects of alpha1-and alpha2-adrenoceptor blockers were evaluated. The results indicate that in the guinea pig bronchi noradrenaline inhibits non-adrenergic, non-cholinergic neurotransmission by acting on prejunctional alpha 2-adrenoceptors.