title: Regulation of sodium transport in the inner ear.
abstract: Na(+) concentrations in endolymph must be controlled to maintain hair cell function since the transduction channels of hair cells are cation-permeable, but not K(+)-selective. Flooding or fluctuations of the hair cell cytosol with Na(+) would be expected to lead to cellular dysfunction, hearing loss and vertigo. This review briefly describes cellular mechanisms known to be responsible for Na(+) homeostasis in each compartment of the inner ear, including the cochlea, saccule, semicircular canals and endolymphatic sac. The influx of Na(+) into endolymph of each of the organs is likely via passive diffusion, but these pathways have not yet been identified or characterized. Na(+) absorption is controlled by gate-keeper channels in the apical (endolymphatic) membrane of the transporting cells. Highly Na(+)-selective epithelial sodium channels (ENaCs) control absorption by Reissner's membrane, saccular extramacular epithelium, semicircular canal duct epithelium and endolymphatic sac. ENaC activity is controlled by a number of signal pathways, but most notably by genomic regulation of channel numbers in the membrane via glucocorticoid signaling. Non-selective cation channels in the apical membrane of outer sulcus epithelial cells and vestibular transitional cells mediate Na(+) and parasensory K(+) absorption. The K(+)-mediated transduction current in hair cells is also accompanied by a Na(+) flux since the transduction channels are non-selective cation channels. Cation absorption by all of these cells is regulated by extracellular ATP via apical non-selective cation channels (P2X receptors). The heterogeneous population of epithelial cells in the endolymphatic sac is thought to have multiple absorptive pathways for Na(+) with regulatory pathways that include glucocorticoids and purinergic agonists.
authors.affiliation: Department of Otorhinolaryngology, Yonsei University College of Medicine, 250 Seongsanno, Seodaemun-gu, Seoul 120-752, Republic of Korea.
authors.forename: Sung Huhn
authors.initials: SH
authors.lastname: Kim
authors.forename: Daniel C
authors.initials: DC
authors.lastname: Marcus
mesh_terms: D000818:Animals; D003051:Cochlea / Q000502:physiology; D007758:Ear, Inner / Q000502:physiology*; D004712:Endolymphatic Sac / Q000502:physiology; D006706:Homeostasis / Q000502:physiology*; D006801:Humans; D017136:Ion Transport / Q000502:physiology; D012444:Saccule and Utricle / Q000502:physiology; D012665:Semicircular Canals / Q000502:physiology; D012964:Sodium / Q000378:metabolism*
chemical_list: D012964:Sodium