Regulation of Na,K-ATPase Function in the Lens

✍ Scribed by Delamere, Nicholas A.

• Publisher: John Wiley and Sons
• Year: 2003
• Tongue: English
• Weight: 111 KB
• Volume: 300A
• Category: Article
• ISSN: 0022-104X
• DOI: 10.1002/jez.a.10305

✦ Synopsis

Abstract

The two cell types in the lens, epithelium and fiber, have a very different specific activity of Na,K‐ATPase; activity is much higher in the epithelium. However, judged by Western blot, fibers and epithelium express a similar amount of both Na,K‐ATPase α and β subunit proteins. Na,K‐ATPase protein abundance does not tally with Na,K‐ATPase activity. Studies were conducted to examine whether protein synthesis plays a role in maintenance of the high Na,K‐ATPase activity in lens epithelium. An increase of cytoplasmic sodium was found to increase Na,K‐ATPase protein expression in the epithelium, but not in the fibers. The findings illustrate the ability of lens epithelium to synthesize new Na,K‐ATPase protein as a way to boost Na,K‐ATPase in response to cell damage or pathological events. Methionine incorporation studies suggested Na,K‐ATPase synthesis may also play a role in day to day preservation of high Na,K‐ATPase activity. Na,K‐ATPase protein in lens epithelial cells appeared to be continually synthesized and degraded. Experiments with cycloheximide suggest that specific activity of Na,K‐ATPase in the lens epithelium may depend on the ability of the cells to continuously synthesize fresh Na,K‐ATPase proteins. However, other factors such as phosphorylation of Na,K‐ATPase α subunit may also influence Na,K‐ATPase activity. When intact lenses were exposed to the agonist thrombin, Na,K‐ATPase activity was diminished, but the response was suppressed by inhibitors of the Src family of non‐receptor tyrosine kinases. Thrombin elicited tyrosine phosphorylation of lens epithelium membrane proteins, including a 100 kDa protein band thought to be the Na,K‐ATPase α 1 subunit. It remains to be determined whether a tyrosine phosphorylation mechanism contributes to the low activity of Na,K‐ATPase in lens fibers. J. Exp. Zool. 300A:25–29, 2003. © 2003 Wiley‐Liss, Inc.