Characterization of 5-hydroxytryptamine release from isolated rabbit and rat trachea: the role of neuroendocrine epithelial cells and mast cells

Abstract

Rabbit or rat isolated tracheae were incubated in vitro, and the release of 5-hydroxytryptamine (5-HT) and its metabolite 5-hydroxyindoleacetic acid (5-HIAA) was determined by HPLC with electrochemical detection. Release of 5-HT from rabbit tracheae could be evoked by the calcium ionophore A 23187 and, in a calcium-dependent manner, by depolarizing concentrations of potassium (45 mmol/l), but not by the mast cell degranulating drug compound 48/80. High potassium- and A 23187-evoked release of 5-HT was markedly higher from tracheae of newborn compared to adult rabbits. In rabbit tracheae, mechanical removal of the mucosa resulted in 80-90% reduction in tissue 5-HT and in a similar reduction in high potassium-evoked 5-HT release. 5-Hydroxytryptophan, but not tryptophan, caused a marked increase in the spontaneous outflow of 5-HT and 5-HIAA from tracheae of newborn rabbits, and the effect on 5-HT, but not that on 5-HIAA, required an intact mucosa. Furthermore, treatment with 5-hydroxytryptophan caused an increase in tissue 5-HT and 5-HIAA, and these effects required an intact mucosa. In tracheae of adult rabbits 5-hydroxytryptophan caused similar, although less profound, effects. Adrenaline (1 micromol/1) enhanced the release of 5-HT from newborn rabbit tracheae, and this effect was inhibited by 1 micro mol/l phentolamine or 1 micromol/1 prazosin, but not affected by 100 nmol/1 propranolol. In rat tracheae, compound 48/80 evoked a large release of 5-HT, whereas depolarizing concentrations of potassium (45 mmol/1) had only a very minor effect. In rat tracheae, 5-hydroxytryptophan had small effects on the outflow and tissue contents of 5-HT and 5-HIAA in comparison to the effects on rabbit tracheae; and removal of the mucosa resulted in only a minor reduction in tissue 5-HT. In conclusion, neuroendocrine epithelial (NEE) cells and mast cells are the major source of 5-HT in tracheae of the rabbit and rat, respectively. Isolated tracheae of newborn rabbits appear to be a useful model to study 5-HT secretion from NEE cells. 5-HT secretion from NEE cells is activated by a rise in intracellular calcium, and calcium influx through voltage-regulated channels appears to be one activating pathway. 5-HT secretion from NEE cells can be stimulated via alpha-adrenoceptors.

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

“…Similar to previous observation (Freitag et al, 1995), in control experiments the outflow of 5-HT ( Figure 1) and that of 5-HIAA (not shown) continuously declined during the observation period. As the outflow of 5-HIAA changed only marginally in response to the various treatments described below the data on 5-HIAA outflow area are not documented in detail.…”

Nitric oxide, via activation of guanylyl cyclase, suppresses α2-adrenoceptor-mediated 5-hydroxytryptamine release from neuroendocrine epithelial cells of rabbit tracheae

“…Similar to previous observations (Freitag et al 1995, the outflow of 5-HT continuously declined during the observation period in control experiments (Fig. 1).…”

“…Recently, evidence was provided that 5-HT release from the isolated rabbit trachea reflects secretion from NEE cells (Freitag et al 1995). In agreement with the morphological observation that the density of NEE cells is particularly high during the perinatal period (Redick and Hung 1984;Cho et al 1989), 5-HT release from tracheae of newborn animals was higher than from tissue of adult animals.…”

“…The animals were killed by stunning followed by exsanguination. Whole tracheae were dissected and incubated as described in detail previously (Freitag et al 1995. Briefly, single preparations were fixed in a glass organ bath and then incubated in 1.4 ml Krebs-HEPES solution of the following composition (mM): NaCl 118.5; KCl 5.7; CaCl 2 1.25; MgCl 2 1.2; Na 2 EDTA 0.03; (+)ascorbic acid 0.06; HEPES 20.0 (adjusted to pH 7.4 using NaOH) and D-glucose 11.1, kept at 37°C and saturated with 100% O 2 .…”

“…5-HT was measured by high pressure liquid chromatography (HPLC) with electrochemical detection as described previously (Freitag et al 1995). HPLC separation was performed by a reverse phase column (length 250 mm, inner diameter 4.6 mm, prepacked with Shadon ODS-Hypersil, 5 µm) using a mobile phase of 0.1 M phosphate buffer (adjusted to pH 3.0), containing octane sulfonic acid sodium salt (160 mg/l), sodium EDTA (0.3 mM) and methanol (12%, v/v).…”