The effect of serotonin on airway transepithelial sodium ion pathways

30-Min Exposure to Tobacco Smoke Influences Airway Ion Transport-An In Vitro Study

Introduction: Smoking is one of the most important causes of cancer in humans. However, it has not been proven how long exposure to cigarette smoke is sufficient to induce cancerogenesis. Cigarette smoke can cause changes in ion and water transport and the maintenance of mucociliary transport. The conducted research concerned the assessment of changes in ion transport in rabbit tracheal specimens after 30 min of exposure to cigarette smoke. Materials and Methods: A modified Ussing chamber was used to measure the transepithelial electrical potential under stationary conditions (PD) and during mechanical stimulation (PDmin), and the transepithelial electrical resistance (R) in control and cigarette smoke-exposed tracheal fragments. Results: Significant changes in PD (-2.53 vs. -3.92 mV) and PDmin (-2.74 vs. -0.39 mV) were noted for the samples exposed to smoke, which can be associated with a rise in reactivity after applying a mechanical stimulus. In addition, the measured R (108 vs. 136 Ω/cm²) indicated no changes in the vitality of the samples, but an increase in their permeability to ions in the experimental conditions. Conclusions: A single 30-min exposure to cigarette smoke has been shown to be associated with increased permeability of the tracheal epithelium to ions and thus to substances emitted during smoking, which might be sufficient to create the possibility of initiating procarcinogenic processes.

Toluene diisocyanate caused electrophysiological disturbances in the upper airways wall

OBJECTIVES

Toluene diisocyanate (TDI) due to its widespread use in industry is one of the most common and well-known causes of occupational asthma and Reactive Airways Dysfunction Syndrome (RADS). In this study the impact of TDI on the electrophysiological properties of the airways wall, particularly on the mechanisms of absorption of sodium ions and chloride ions secretion was evaluated.

MATERIALS AND METHODS

Isolated rabbit tracheal wall (from outbred stock animals) was mounted in an apparatus for electrophysiological experiments by means of Ussing method and was mechanically stimulated by the jet flux of specified fluid directed onto the mucosal surface of the tissue from a peristaltic pump. The measured parameters were: transepithelial potential difference under control conditions (PD, mV), after mechanical stimulation (dPD or physiological reaction of hyperpolarization, mV) and electric resistance (R, Omega cm2). When TDI (0.035 mM) was added to stimulation fluid, only the immediate reaction was identified and when it was added to incubation fluid and other experimental fluids, the late (post-incubation) reaction was determined. The experiments involving the inhibition of Na+ by amiloride and Cl- by bumetanide were also performed.

RESULTS

A series of functional tests for 72 pieces of tracheal wall from 36 animals were performed. It has been shown that short-term exposure to TDI significantly changed the course of reactions to mechanical stimulation. Also after incubation in the presence of TDI, the reactions to mechanical stimulation were changed in relation to control conditions.

CONCLUSIONS

The immediate reaction of the isolated rabbit tracheal wall after exposure to TDI depends on the duration of exposure and on the physiological condition of the tissue in respect of sodium and chloride ion transport.

Serotonin decreases alveolar epithelial fluid transport via a direct inhibition of the epithelial sodium channel

Hypoxia and epithelial stretch that are commonly observed in patients with acute lung injury have been shown to promote the release of serotonin (5-hydroxytryptamine, 5-HT) in vitro. However, whether 5-HT contributes to the decrease of alveolar epithelial fluid transport, which is a hallmark of lung injury, is unknown. Thus, we investigated the effect of 5-HT on ion and fluid transport across the alveolar epithelium. 5-HT caused a dose-dependent inhibition of the amiloride-sensitive current across primary rat and human alveolar epithelial type II cell monolayers, but did not affect Na(+)/K(+) ATPase function. Furthermore, we found that the 5-HT induced inhibition of ion transport across the lung epithelium was receptor independent, as it was not prevented by the blockade of 5-HT2R (5-HT receptor 2), 5-HT3R (5-HT receptor 3), or by pretreatment with an intracellular calcium-chelating agent, BAPTA-AM (1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid tetra(acetoxymethyl) ester). In addition, the stimulation of 5-HT1R (5-HT receptor 1), 5-HT2R (5-HT receptor 2), 5-HT4R (5-HT receptor 4), and 5-HT7R (5-HT receptor 7) failed to reproduce the 5-HT effect on amiloride-sensitive sodium transport. We ascertained that 5-HT directly inhibited the function of rat alphabetagamma epithelial sodium channel (ENaC), as determined by heterologous expression of rat ENaC in Xenopus oocytes that do not express endogenous ENaC nor 5-HT receptors (5-HTR). Exposure of mice to hypoxia for 1 hour induced a 30% increase of 5-HT secretion into the distal airways of mice. Finally, the intratracheal instillation of 5-HT inhibited the amiloride-sensitive fraction of alveolar fluid clearance in mice. Together, these results indicate that 5-HT inhibits the amiloride-sensitive fraction of the alveolar epithelial fluid transport via a direct interaction with ENaC, and thus can be an endogenous inhibitor of this ion channel.