Expression patterns of ion transport enzymes in spiral ligament fibrocytes change in relation to strial atrophy in the aged gerbil cochlea

Fibrocytes of the lateral wall function in conjunction with the stria vascularis (StV) to mediate cochlear ion homeostasis. Age related changes in the expression patterns of ion transport enzymes in spiral ligament fibrocytes were investigated to ascertain their relation to metabolic presbyacusis in the gerbil. Immunoreactivity of fibrocytes for Na,K-ATPase (Na,K), carbonic anhydrase isozyme II (CA) and creatine kinase isozyme BB (CK) varied among and within cochleas from aged but not from young gerbils. The variable immunostaining was related to the extent and location of StV atrophy. Age-dependent degeneration and loss of Na,K in the StV occurred predominantly in the apex and lower base and hook of the cochlea, largely sparing more central regions. Immunostaining intensity for Na,K, CK, and CA in fibrocytes changed in relation to declines in strial marginal cell Na,K initially showing upregulation followed by downregulation. Spiral ligament fibrocytes in cochleas with more than two remaining normal turns often disclosed overexpression of CK in regions of strial atrophy. Conversely, CA in such cochleas was often increased in regions of normal StV adjacent to foci of atrophic StV. Senescent cochleas with two or fewer functional turns generally contained fibrocytes with diminished CK or CA immunoreactivity in regions of atrophic StV but in isolated instances exhibited fibrocytes with enhanced staining. Heightened staining for CK in type Ia fibrocytes underlying regions of complete or partial strial atrophy indicated an increased metabolic demand in fibrocytes in response to strial insufficiency. The findings provide further support for the role of spiral ligament fibrocytes in cochlear fluid and ion homeostasis.