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(Investigative Ophthalmology and Visual Science. 2003;44:4395-4399.)
© 2003 by The Association for Research in Vision and Ophthalmology, Inc.
DOI:  10.1167/iovs.03-0287
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Regional Distribution of Na,K-ATPase Activity in Porcine Lens Epithelium

Shigeo Tamiya,1 William L. Dean,2 Christopher A. Paterson,1 and Nicholas A. Delamere1,3

1From the Departments of Ophthalmology and Visual Sciences, 2Biochemistry and Molecular Biology, and 3Pharmacology and Toxicology, University of Louisville, School of Medicine, Louisville, Kentucky.

PURPOSE. It has been established that Na,K-ATPase activity is higher in lens epithelium than fibers. However, others have suggested the Na,K-ATPase enzyme may be inactive or absent in the central 10% of the epithelium. Studies were conducted to measure and compare Na,K-ATPase specific activity and to examine Na,K-ATPase protein expression in the anterior and equatorial regions of porcine lens epithelium.

METHODS. Na,K-ATPase activity was determined by measuring the ouabain-sensitive rate of adenosine triphosphate (ATP) hydrolysis. Western blot analysis was used to detect Na,K-ATPase catalytic subunit ({alpha}) and glycoprotein subunit (ß) protein as well as ß-actin which was used as a loading control.

RESULTS. Na,K-ATPase specific activity was more than two times higher in the equatorial epithelium than the anterior 50% of the epithelium. However, the abundance of Na,K-ATPase {alpha}1 isoform protein was similar in the two regions. Neither the {alpha}2 nor {alpha}3 Na,K-ATPase isoform could be detected in the anterior or equatorial epithelium, but Na,K-ATPase ß1 protein was detected in both regions. In contrast to the observed regional difference in Na,K-ATPase activity, the activity of a different P-type ATPase, plasma membrane Ca-ATPase (PMCA), was not significantly different in the anterior and central epithelium. Western blot analysis indicated the presence of two PMCA isoforms, PMCA2, and PMCA4.

CONCLUSIONS. Na,K-ATPase activity is significantly higher at the equatorial region of the epithelium compared with the anterior, even though the level of Na,K-ATPase protein is similar in the two regions. It is possible that nonuniform distribution of functional Na,K-ATPase activity contributes to the driving force for circulating solute movement through the lens fiber mass.




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