Observations on the electrochemistry of the cochlear endolymph of the rat: a quantitative study of its electrical potential and ionic composition as determined by means of flame spectrophotometry

Abstract

The relationship between the high positive potential and the high potassium and low sodium concentrations within the endolymph has been investigated in the adult rat. Very small, (2 nl.) uncontaminated samples of cochlear endolymph and perilymph have been collected and the endolymph potential measured at the times of collection. The sodium and potassium contents of the samples were estimated by means of total emission, integrative flame spectrophotometry. In the course of the procedure a number of serious problems were encountered, in particular that arising from the extremely small sodium content of the endolymph. For their solution a number of technical improvements were required, including the development of a new type of burner. A measure of the sensitivity thus attained is provided by the finding that, using test samples containing 4.8 x 10 -12 mequiv. (1.1 x 10 -13 g) of sodium, the standard deviation of the analytical results was ± 8*4 x 10 -13 mequiv. (±17.6%). With a solution comparable in composition to endolymph the standard deviation was ± 6% for sodium and ±1.3% for potassium. The analytical results showed the values of the sodium and potassium concentrations in the endolymph to be 0.91 and 154 mequiv./I. respectively. In perilymph these values were 138 and 6.9 mequiv./l. The average endolymphatic potential was +92 mV. During anoxia the positive endolymphatic potential was replaced by a negative potential reaching, on average, a maximum of - 42 mV after 4 1/2 min and thereafter slowly returning to zero. The principal ionic changes were a progressive increase in the endolymphatic sodium concentration from 3.6 mequiv./l. after 2 min anoxia to 32 mequiv./l. after 30 min anoxia and an associated decrease in the endolymphatic potassium concentration to 116 mequiv./l. after 30 min anoxia. These results establish that the low sodium content of the endolymph is maintained by means of an active transport mechanism which is probably situated in the stria vascularis. It thus appears that the characteristic composition of the endolymph is due to the active transfer of sodium and chloride from and potassium into the cochlear duct and that the mechanisms concerned are highly dependent on oxidative metabolism. The possible interrelationship of these mechanisms and the origin of the endolymphatic potential are briefly discussed but are considered to be still obscure.