Oxygen metabolites modulate sodium transport in gerbil middle ear epithelium: involvement of PGE2

Superoxide enhances Na-K-2Cl cotransporter activity in the thick ascending limb

Superoxide (O2-) enhances Na reabsorption by the thick ascending limb (THAL). Na absorption in this segment involves the Na-K-2Cl cotransporter, K channel, and Na-K-ATPase. We hypothesized that O2- stimulates NaCl absorption primarily by enhancing Na-K-2Cl cotransport. First, we measured steady-state intracellular Na (Nai) and chloride (Cli). Xanthine oxidase (XO; 0.75 mU/ml) and hypoxanthine (HX; 0.125 mM) were added to the bath to increase O2-. During the control period, Nai was 12.2 +/- 1.9 mM. After treatment with O2-, it rose to 20.9 +/- 3.3 mM, a 71% increase (P < 0.01). Cli also increased (P < 0.01). Neither XO nor HX alone had a significant effect on Nai or Cli. Next, we tested cotransport activity by measuring the initial rate of increase in Nai caused by changing luminal Na-Cl-K from 50/0/0 to 140/134/4 mM. During the control period, the initial rate of increase was 0.13 +/- 0.02 arbitrary units (AU)/min. After treatment with O2-, it was 0.22 +/- 0.04 AU/min (P < 0.025), a 69% increase. Neither XO nor HX alone had a significant effect. Furosemide completely blocked the increase in intracellular Na in the control and O2- treatment periods. Next, we studied K channel activity by measuring the depolarization caused by increasing luminal K from 1 to 25 mM using a voltage-sensitive dye. During the control period, maximum depolarization was 7.31 +/- 0.77 AU. After O2- treatment, it was 6.18 +/- 0.90 AU (P < 0.05), a 15% decrease. Finally, we assessed the effects of O2- on Na-K-ATPase activity in THAL suspensions by measuring ATP hydrolysis. Vmax and K1/2 for Na were not affected by O2-. We concluded that O2- stimulates THAL NaCl absorption primarily by enhancing Na entry via Na-K-2Cl cotransport.

Serous or mucoid effusion in the course of secretory otitis media: influence of ion transport modulation

OBJECTIVE

Secretory otitis media is defined by a chronic effusion in the middle ear cavities, behind an intact tympanic membrane without acute infection. In the course of secretory otitis media, the effusion is sometimes thick (or mucoid) and other times thin (or serous). For several authors, these differences might be related to different inflammatory levels rather than to distinct pathophysiological mechanisms. The purpose of this study was to determine whether the modulation of ion transport by inflammation could account for the differences observed in the aspect of ME effusion.

METHODS

Hydrogen peroxide (H2O2), used as an inflammation model was tested on a middle ear cell line (MESV) in culture.

RESULTS

Results show that low and high concentrations of H2O2 have opposite effects on ion (and then water) absorption by the middle ear epithelium.

CONCLUSION

the modulation of ion and water absorption by inflammation could modify the mucins concentration in the effusion, and explain observed viscosity differences in the course of secretory otitis media.

Inflammatory mediators and otitis media with effusion. An experimental approach using cell culture

Otitis media with effusion is characterized by the presence of an inflammatory cellular infiltrate of the submucosa and a poor ventilation of the middle ear. This result in hypersecretion of mucus and alteration of the mucociliary clearance, which produce accumulation of fluid and cellular debris in the middle ear. The aim of this work was to investigate whether inflammatory mediators such as prostaglandins and oxygen metabolites modulate the absorptive function of the middle ear epithelium. The data we present demonstrated that: (i) among prostanoids, only prostaglandin E2 modulated the rate of sodium transport; (ii) oxidants had a stimulatory effect on ion transport; (iii) the role of reactive oxygen species was mediated by prostaglandin E2. This process might be involved in the impairment of the mucociliary clearance.