Very low calcium content of cochlear endolymph, an extracellular fluid

Abstract

IONIC calcium is important in many membrane functions and the possibility of it being involved in the cochlear transduction processes is supported by the scanty evidence available¹. In the analogous lateral line organ of submammalian species, the mechanosensitivity of the hair cells is related directly to the external calcium concentration at their specialised receptor poles². There is also evidence of interaction with monovalent cation effects^2,3. In mammals, the cochlear duct is filled with endolymph, the unusual constitution of which (in the rat d.c. potential + 88 mV, K⁺ 148 mM, Na⁺ 0.84 mM) is seemingly maintained entirely by active transport mechanisms⁴ and is generally conceded to be necessary for normal transduction. We report here that the cochlear endolymph calcium occurs almost entirely in the non-complexed state at a concentration of 3×10⁻⁵ M in the rat. This is substantially below the usual extracellular concentration, and must influence the function of the receptor pole membranes and might well affect the role of calcium in the transduction process in the cochlea. The total magnesium concentration we measured was 1×10⁻⁵ M.