Overflow of endogenous noradrenaline (NA) from the in vitro incubated rat trachea evoked by two periods of electrical field stimulation (S1, S2, at 3 or 15 Hz) or by high potassium (60 mm) was determined by high performance liquid chromatography (h.p.l.c.) with electrochemical detection.
In the presence of the neuronal uptake inhibitor desipramine, the α2‐adrenoceptor antagonist, yohimbine, enhanced the overflow of NA evoked by stimulation at 3 Hz by about 100% suggesting the presence of presynaptic inhibitory autoreceptors on the sympathetic nerves innervating the trachea.
When desipramine and yohimbine were present throughout the experiments, the overflow of NA evoked by the second period of electrical stimulation (S2) was significantly smaller than that evoked by the first (S1). This decline of overflow was prevented when the NA precursor, tyrosine, was additionally present throughout the experiments.
After removal of the epithelium, the tissue content of NA was reduced by about 30%, suggesting that part of the NA may be present and released within the epithelium. However, the overflow of NA evoked by stimulation at 3 Hz or 15 Hz was reduced by 70–80%, indicating that the epithelium may additionally exert a permissive role on the release of NA within the airways, possibly by suppressing inhibitory factors.
Stimulation by high potassium (60 mm for 10 min) caused a large overflow of NA (about 45% of the tissue NA), both from epithelium‐free and epithelium‐denuded tracheae. Thus the ‘endogenous inhibition’ of NA release after removal of the epithelium is surmountable when a high potassium stimulus is applied.
An enhanced muscarinic inhibition of NA release in epithelium‐denuded tracheae may not be responsible for the reduction in NA overflow, since the muscarinic receptor antagonist, scopolamine, did not significantly affect the overflow of NA evoked by 3 or 15 Hz, either in the presence or the absence of the epithelium.
The muscarinic receptor agonist, oxotremorine, inhibited the overflow of NA evoked by stimulation at 3 Hz almost completely, an effect antagonized by scopolamine, demonstrating specific, inhibitory muscarinic receptors.
Since damage to the epithelium is known to occur in obstructive bronchial disease, the present experiments suggest that an impaired sympathetic (broncho‐relaxant) neurotransmission could be involved in the pathogenesis of bronchoconstriction.
