Ionic Composition of Endolymph in Teleosts: Origin and Importance of Endolymph Alkalinity

Abstract

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.

Discriminant inter and intrapopulation variation in sagittal otolith shape and morphometry in Chelon ramada (Actinopterygii, Mugilidae) from the Boughrara and El Bibane lagoons in Tunisian waters

“…These dense otolith structures are the asterisci (otoliths of the lagena), lapilli (otoliths of the utriculus), and sagittae (otoliths of the sacculus) and are closely associated with the auditory sensory epithelium, function in the transmission of sound, and are also involved in maintaining a static and active balance (Parmentier et al ., 2001). Of these otoliths, the sagittal otoliths or sagittae are the largest of them, found just behind the eyes and approximately level with them vertically, while the lapilli and asterisci are the smallest and are located within the utriculus and lagena chambers, respectively, which are continuous with three semi-circular canals (Payan et al ., 1997; Schwarzhans et al ., 2017). Interestingly, the acoustic properties (density and elasticity) of fish tissue are very similar to the surrounding water, and when the fish are exposed to sound, the otoliths function as an accelerometer (Zhang et al ., 2021).…”

Immunohistochemical and ultrastructural characterization of the inner ear epithelial cells of splitnose rockfish (Sebastes diploproa)

“…The meshwork area is known to have T1-Is and T2-Is (Figure 1). The T1-Is are known to be mitochondrion-, NKA-and NKCC-rich (24,(36)(37)(38), have short thick microvilli (18), and thought to be responsible maintaining the ionic and acid-base parameters of the endolymph (19,24,37,39,40). T1-Is are interconnected and together form intracytoplasmic membrane systems, and their abundance decreases with distance away from the macula (18,38).…”

“…Since otoliths are formed inside the inner ear endolymph and are not in direct contact with the water, the altered biomineralization pattern and the accumulation of trace element and isotopes within the polycrystal necessitates a degree of biological regulation by the inner ear epithelial cells. Indeed, the endolymph has higher average pH, [HCO3 − ], [CO3 2− ], total CO2, and [K + ] than the blood plasma (24,39,40,54,55).…”

Immunohistochemical and ultrastructural characterization of the inner ear epithelial cells of splitnose rockfish (Sebastes diploproa)

“…Since otoliths are formed inside the inner ear endolymph and are not in direct contact with the water, the altered biomineralization pattern and the accumulation of trace element and isotopes within the polycrystal necessitates a degree of biological regulation by the inner ear epithelial cells. Indeed, the endolymph has higher average pH, [HCO 3 −], [CO 3 2− ], total CO 2 , and [K + ] than the blood plasma (24, 39, 40, 54, 55). Moreover, the endolymph in the proximal side of the inner ear organ (facing the macula) has much lower [K + ] and pH than the distal side (facing the patches area) (19, 40), a chemical heterogeneity that is most likely generated by the activity of the inner ear epithelial cells in each region.…”