Water permeability of the mammalian cochlea: functional features of an aquaporin-facilitated water shunt at the perilymph–endolymph barrier

Abstract

The cochlear duct epithelium (CDE) constitutes a tight barrier that effectively separates the inner ear fluids, endolymph and perilymph, thereby maintaining distinct ionic and osmotic gradients that are essential for auditory function. However, in vivo experiments have demonstrated that the CDE allows for rapid water exchange between fluid compartments. The molecular mechanism governing water permeation across the CDE remains elusive. We computationally determined the diffusional (P D) and osmotic (P f) water permeability coefficients for the mammalian CDE based on in silico simulations of cochlear water dynamics integrating previously derived in vivo experimental data on fluid flow with expression sites of molecular water channels (aquaporins, AQPs). The P D of the entire CDE (P D = 8.18 × 10−5 cm s−1) and its individual partitions including Reissner's membrane (P D = 12.06 × 10−5 cm s−1) and the organ of Corti (P D = 10.2 × 10−5 cm s−1) were similar to other epithelia with AQP-facilitated water permeation. The P f of the CDE (P f = 6.15 × 10−4 cm s−1) was also in the range of other epithelia while an exceptionally high P f was determined for an epithelial subdomain of outer sulcus cells in the cochlear apex co-expressing AQP4 and AQP5 (OSCs; P f = 156.90 × 10−3 cm s−1). The P f/P D ratios of the CDE (P f/P D = 7.52) and OSCs (P f/P D = 242.02) indicate an aqueous pore-facilitated water exchange and reveal a high-transfer region or “water shunt” in the cochlear apex. This “water shunt” explains experimentally determined phenomena of endolymphatic longitudinal flow towards the cochlear apex. The water permeability coefficients of the CDE emphasise the physiological and pathophysiological relevance of water dynamics in the cochlea in particular for endolymphatic hydrops and Ménière's disease.