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 mu M) enhanced the outflow of 5-HT by 80, 290 and 205%, respectively. 5-HT outflow evoked by 10 mu M phenylephrine was not affected by the presence of the neurotoxin tetrodotoxin (1 mu M). 3 Rauwolscine, ARC 239 (an alpha(2B)-adrenoceptor preferring antagonist), yohimbine and prazosin antagonized the effect of 10 mu M 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 K-i 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 mu M phenylephrine was inhibited by 65% in the presence of 1 mu M forskolin and abolished in the presence of 10 mu M forskolin. 5 5-HT outflow evoked by 10 mu M phenylephrine was inhibited by about 45 and 70% in the presence of 0.1 and 1 mu M isoprenaline, respectively. The inhibitory effect of 1 mu M isoprenaline was only marginally antagonized by 1 mu M, but blocked by 10 mu M propranolol. 6 5-HT outflow was not affected by the muscarine receptor agonist oxotremorine (10 mu M), but was enhanced by 175% by 100 mu M nicotine. The effect of nicotine was blocked by 100 mu M hexamethonium and prevented by 1 mu M tetrodotoxin or 1 mu M 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.