Evidence for non-cholinergic, non-adrenergic nervous control of mucus secretion into the cat trachea

We have investigated the effects of electrical stimulation of the vagus nerves on the output of mucus glycoproteins (mucins), radiolabelled with 3H and 35S, into the trachea of anaesthetized cats. In five control experiments, stimulation of the vagus nerves on four successive occasions, separated by 1 h, caused significant rises in the output of radiolabelled mucins. In these experiments repetition of stimulation did not appear to lessen the response. In a parallel series of five experiments the vagus nerves were again stimulated on four occasions, but atropine was administered in increasing doses between the stimuli. Large responses, not significantly less than those seen in the corresponding control stimulations, were seen even in the presence of the highest dose of atropine. In this series of experiments, however, the effect of the last vagal stimulation (with the highest dose of atropine) was significantly less then the first (no atropine). Administration of phentolamine and l-propranolol in addition to atropine failed to reduce the response to vagal stimulation significantly. We conclude that, while cholinergic nerves can probably explain part of the increase in mucin output which occurs with vagus nerve stimulation, there is a large response mediated by a non-cholinergic, non-adrenergic neurotransmitter. Possible neurotransmitters and the relationship of these findings to those of earlier studies are discussed.