Ionic composition of endolymph in teleosts: origin and importance of endolymph alkalinity

Ionic (Na+, K+, Cl-, PO4(3-), pH), total CO2, total calcium and protein concentrations in the plasma and endolymph of the inner ear were compared in trout Oncorhynchus mykiss and turbot Scophthalmus maximus. In both species, saccular endolymph was characterized by high levels of K+ and total CO2 and in trout by an alkaline pH. The kinetic characteristics of proton secretion across the saccular epithelium of trout were investigation using a titration technique in which isolated saccules were mounted as closed sacs. The rate of proton secretion depends strongly on the pH of the Ringer's solution and secretion stops at a pH below 7.2. Proton secretion is driven by an energy-dependent mechanism involving basolateral ouabain-sensitive Na+/K+ exchangers. Proton secretion was partially inhibited by acetazolamide and completely inhibited in Na(+)-free Ringer or in the presence of 1 mmol l-1 amiloride. A cellular model stressing the importance of proton exchange through the saccular epithelium is proposed to explain the regulation of endolymph pH, a crucial factor for the deposition of otolith calcium.