Regional distribution of Na,K-ATPase activity in porcine lens epithelium.
OBJECTIVE
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 (beta) protein as well as beta-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 alpha1 isoform protein was similar in the two regions. Neither the alpha2 nor alpha3 Na,K-ATPase isoform could be detected in the anterior or equatorial epithelium, but Na,K-ATPase beta1 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.
D000255:Adenosine Triphosphate / Q000378:metabolism; D000718:Anatomy, Regional; D000818:Animals; D015153:Blotting, Western; D000252:Calcium-Transporting ATPases / Q000378:metabolism; D002462:Cell Membrane / Q000201:enzymology; D004848:Epithelium / Q000201:enzymology; D007527:Isoenzymes / Q000378:metabolism; D007908:Lens, Crystalline / Q000166:cytology / Q000201:enzymology*; D010042:Ouabain; D000254:Sodium-Potassium-Exchanging ATPase / Q000378:metabolism*; D013552:Swine
D007527:Isoenzymes; D010042:Ouabain; D000255:Adenosine Triphosphate; D000252:Calcium-Transporting ATPases; D000254:Sodium-Potassium-Exchanging ATPase