Oxygen modulates Na+ absorption in middle ear epithelium

The physiology of the middle ear is primarily concerned with keeping the ca vities air filled and fluid free to allow transmission of the sound vibrati ons from the eardrum to the inner ear. Middle ear epithelial cells are thou ght to play a key role in this process, since they actively transport-Na+ a nd water. The P-O2 of the middle ear cavities varies from 44 to 54 mmHg in healthy human ears but may be lower in the course of secretory otitis media . The effect of chronic hypoxia on ion transport was investigated on a midd le ear cell line using the short-circuit current technique. Chronic hypoxia reversibly decreased the rate of Na+ absorption across the MESV cell Line. Although a decrease in cellular ATP content was observed, the decrease of Na+ absorption seemed related to a primary modulation of apical Na+ entry. As revealed by RNase protection assay, the decrease in the rate of apical N a+ entry strictly paralleled the decrease in the expression of transcripts encoding the alpha-subunit of the epithelial Na+ channel. This effect of ox ygen on Na+ absorption might account for 1) the presence of fluid in the mi ddle ear in the course of secretory otitis media and 2) the beneficial effe ct of the ventilation tube in treating otitis media that allows the P-O2 to rise and restores the fluid clearance.