Lateral wall Na,K-ATPase and endocochlear potentials decline with age in quiet-reared gerbils

Abstract

Changes in the integrity of cochlear ion transport systems with age were examined in gerbils raised for 5-38 months in a quiet environment. Ion transport function was assessed by light microscopic immunohistochemical staining for the enzyme, Na,K-ATPase and by measurement of the endocochlear potential (EP). Small foci of strial atrophy accompanied by loss of immunostaining for Na,K-ATPase were observed in the stria vascularis of the apical and basal turns as early as 5 months of age. Cochleas from 29-38 month-old gerbils showed a loss of immunostaining for Na,K-ATPase in the stria in most of the apical turn with the degeneration extending well into the middle turn in many of the oldest ears. The extent of strial atrophy and loss of immunoreactive Na,K-ATPase in the basal turn varied considerably among the oldest cochleas. Populations of lateral wall fibrocytes (type II fibrocytes) normally rich in Na,K-ATPase exhibited a corresponding decrease in enzyme content in regions of advanced strial atrophy. The volume of immunostained stria vascularis correlated well with the magnitude of the resting EP. The results demonstrate that lateral wall ion transport systems in the gerbil cochlea degenerate as a function of age. The findings also provide good evidence for a functional relationship between the stria vascularis and the Na,K-ATPase-rich type II fibrocytes in generating and maintaining the EP.