Development of Monovalent Ions in the Endolymph in Mouse Cochlea

The present study was designed to clarify the chronological developmental process of monovalent ions (Na(+), K(+), Cl(-)) in the endolymph of the mouse in relation to the development of the endocochlear potential (EP). The EP and ionic concentrations were measured simultaneously with the ion-sensitive double-barreled microelectrodes from the scala media of the basal turn. The EP increased abruptly 7 days after birth (DAB) and reached approximately 80 mV 14 DAB. In the earliest postnatal days, the endolymphatic Na(+) concentration was significantly higher than that in adult mice, however, the K(+) and the Cl(-) concentrations were lower. The concentrations of all the monovalent ions in endolymph reached adult levels at 7 DAB when the EP was still under 20 mV. These data strongly suggest the presence of a different mechanism between the production of monovalent ions, especially of high K(+) in the endolymph and that of EP.