Occurrence of NaK-ATPase isoforms during rat inner ear development and functional implications

Abstract

This study examined the presence of NaK-ATPase isoforms in the developing inner ear of the rat and studied the importance of functional subunit combinations in endolymph homeostasis. The findings were: (a) the combination α₁β₁ is found in epithelial, mesenchymal, and neural inner ear cells with an early starting expression 14 days postconception (dpc) in some endolymphatic sac cells; (b) from 1 day after birth (dab) expression of α₁β₂ is observed in marginal cells, vestibular dark cells, and certain vestibular nonsensory cells; (c) a transient expression of α₂β₁ is found in suprastrial fibrocytes and spiral ligament fibrocytes type II between 10 and 15 dab; (d) starting at 16 dpc the combination α₃β₁ is uniquely expressed in inner ear neural cells (as in other neural tissues). In conclusion, during development a switch from α₂β₁ towards α₁β₁ is observed in suprastrial fibrocytes and in spiral ligament fibrocytes type II. Thus, according to the biochemical characteristics of these combinations, a switch towards a NaK-ATPase with higher capacity takes place. In addition, prominent expression of the α₁β₂ combination in predominantly K⁺ ion transporting marginal and dark cells is in accordance with the characteristic of this combination and thus with the presumed function of these cells as important K⁺ suppliers for the endolymph. We believe this combination in certain vestibular nonsensory cells to be involved in K⁺ sensing. Early expression of the α₁β₁ combination in the endolymphatic sac, prior to that in the other parts of the inner ear, suggests that this structure may be involved to some extent in the development of the vestibulum and cochlea.