NA,K-ATPASE ACTIVITY DECREASES IN THE COCHLEAR LATERAL WALL OF QUIET-AGED GERBILS

Alterations in the distribution and activity of Na,K-ATPase have been implicated in declining cell function with age. However, the location, size and anatomical complexity of the cochlea have limited study of t his essential enzyme. Here we describe a micro-colorimetric assay whic h measures Na,K-ATPase activity in subregions of individual cochleae. Na,K-ATPase activity was determined in lateral wall and organ of Corti tissues by measuring liberation of inorganic phosphate (P-i) from ATP against a standard phosphate curve. Na,K-ATPase specific activity, ex pressed as mu mol P-i liberated/mg protein/h, was calculated as the di fference between total P-i liberated versus P-i liberated in the prese nce of ouabain. Na,K-ATPase specific activity and total protein conten t in the lateral wall significantly exceeded those of the organ of Cor ti. Although lateral wall protein content remained constant with age, Na,K-ATPase specific activity declined in some older gerbils, suggesti ng a basis for age-related reductions in magnitude of the endocochlear potential and confirming previous histochemical results. This microas say offers a sensitive, reliable means to assay enzyme activity in sub regions or single turns of the cochlea that unlike other methods does not rely on use of radioisotopes, enzymatic cycling or sample pooling.