title: Na,K-ATPase activity decreases in the cochlear lateral wall of quiet-aged gerbils.
abstract: 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 this essential enzyme. Here we describe a micro-colorimetric assay which 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 (Pi) from ATP against a standard phosphate curve. Na,K-ATPase specific activity, expressed as mu mol Pi liberated/mg protein/h, was calculated as the difference between total Pi liberated versus Pi liberated in the presence of ouabain. Na,K-ATPase specific activity and total protein content in the lateral wall significantly exceeded those of the organ of Corti. Although lateral wall protein content remained constant with age, Na,K-ATPase specific activity declined in some older gerbils, suggesting a basis for age-related reductions in magnitude of the endocochlear potential and confirming previous histochemical results. This microassay offers a sensitive, reliable means to assay enzyme activity in subregions or single turns of the cochlea that unlike other methods does not rely on use of radioisotopes, enzymatic cycling or sample pooling.
authors.affiliation: Department of Otolaryngology and Communicative Sciences, Medical University of South Carolina, Charleston 29425-2242, USA.
authors.forename: M A
authors.initials: MA
authors.lastname: Gratton
authors.forename: B J
authors.initials: BJ
authors.lastname: Smyth
authors.forename: B A
authors.initials: BA
authors.lastname: Schulte
authors.forename: D A
authors.initials: DA
authors.lastname: Vincent
mesh_terms: D000375:Aging / Q000378:metabolism*; D000704:Analysis of Variance; D000818:Animals; D002138:Calibration; D002463:Cell Membrane Permeability; D002478:Cells, Cultured; D003124:Colorimetry; D005260:Female; D005849:Gerbillinae; D006801:Humans; D007687:Kidney Tubules, Proximal / Q000166:cytology; D008297:Male; D009925:Organ of Corti / Q000166:cytology / Q000201:enzymology*; D010710:Phosphates / Q000378:metabolism; D000254:Sodium-Potassium-Exchanging ATPase / Q000378:metabolism*; D013056:Spectrophotometry, Ultraviolet; D013316:Stria Vascularis / Q000166:cytology / Q000201:enzymology*; D013701:Temporal Bone / Q000166:cytology
chemical_list: D010710:Phosphates; D000254:Sodium-Potassium-Exchanging ATPase