Adrenoceptor- and cholinoceptor-mediated mechanisms in the regulation of 5-hydroxytryptamine release from isolated tracheae of newborn rabbits.

1. Isolated tracheae of newborn rabbits were incubated in vitro and the outflow of 5-hydroxytryptamine (5-HT) was determined by h.p.l.c. with electrochemical detection. Evidence has previously been provided that this 5-HT outflow derives from neuroendocrine epithelial (NEE) cells of the airway mucosa. 2. Phenylephrine (1, 10 and 30 microM) enhanced the outflow of 5-HT by 80, 290 and 205%, respectively. 5-HT outflow evoked by 10 microM phenylephrine was not affected by the presence of the neurotoxin tetrodotoxin (1 microM). 3. Rauwolscine, ARC 239 (an alpha(2B)-adrenoceptor preferring antagonist), yohimbine and prazosin antagonized the effect of 10 microM phenylephrine in a concentration-dependent manner with IC50 values of 150, 295, 300 and 1,700 nM, respectively. Comparison of the ratios (between all antagonists) of the present IC50 values with the corresponding ratios of Ki values obtained in binding studies for the alpha(2A)-, alpha(2B)-, alpha(2C)- and alpha(2D)-adrenoceptor subtypes strongly suggests the involvement of an alpha(2B)-receptor. 4. 5-HT outflow evoked by 10 microM phenylephrine was inhibited by 65% in the presence of 1 microM forskolin and abolished in the presence of 10 microM forskolin. 5. 5-HT outflow evoked by 10 microM phenylephrine was inhibited by about 45 and 70% in the presence of 0.1 and 1 microM isoprenaline, respectively. The inhibitory effect of 1 microM isoprenaline was only marginally antagonized by 1 microM, but blocked by 10 microM propranolol. 6. 5-HT outflow was not affected by the muscarine receptor agonist oxotremorine (10 microM), but was enhanced by 175% by 100 microM nicotine. The effect of nicotine was blocked by 100 microM hexamethonium and prevented by 1 microM tetrodotoxin or 1 microM yohimbine. 7. In conclusion, 5-HT release from NEE cells of the rabbit trachea is stimulated via alpha-adrenoceptors most likely of the alpha(2B)-subtype localized directly at the NEE cells. Activation of beta-adrenoceptors as well as direct activation of adenylyl cyclase by forskolin exert inhibitory effects on 5-HT release. Activation of nicotinic, but not of muscarinic receptors, also evokes the release of 5-HT. However, the effect of nicotine appears to be mediated indirectly via the release of noradrenaline.