Isoforms of Na,K-ATPase in rat lens epithelium and fiber cells

Purpose. The lens epithelium is thought to conduct Na-K transport for the entire lens cell mass, Lens fibers have a poor ion transport capacity. The authors tested whether different Na,K-ATPase polypeptides are expressed in the two cell types and whether both cells have the machinery needed for ongoing Na,Ii-ATPase expression as judged by the presence of mRNA for the Na,K-ATPase alpha subunit. Methods. Membranes were isolated from adult rat lens epithelium or fibers, and Western blot experiments were conducted for Na,K-ATPase alpha(1), alpha(2), and alpha(3) polypeptides. Total RNA was isolated from adult rat lens epithelium or fiber cells, and Northern analysis was conducted for Na,K-ATPase alpha(1), alpha(2), and alpha(3) mRNA. Some experiments were conducted using fiber cells from neonatal (3-day-old) rat lenses. Results. Multiple isoforms of Na,K-ATPase were detected in adult rat lens epithelium. Judged by Northern blot band intensity, mRNA for Na,K-ATPase alpha(1) and alpha(2) was more abundant than for alpha(3) mRNA, By Western blot, Na,K-ATPase alpha(1), alpha(2), and alpha(3) polypeptides were observed as sharp bands at 100 to 108 kDa. In fiber cells, only Na,R-ATPase alpha(1) immunoreactive polypeptide was detected. Judged by immunoblot density, the amount of alpha(1) polypeptide was similar in both epithelium and fiber cell material. However, Na,K-ATPase alpha subunit mRNA. was not found in adult lens fibers. To test whether Na,K-ATPase synthesis takes place during fiber cell growth, Northern blot analysis was conducted with RNA from neonatal (3-day-old) lens fibers; Na,K-ATPase alpha(1) mRNA was clearly visible. Conclusions. Adult rat lens epithelium expresses more than one isoform of Na,K-ATPase catalytic subunit, whereas only the alpha(1) isoform can be detected in fiber cells. In adult rat lens fiber cells, the observation of alpha(1) polypeptide, but no alpha(1) mRNA, suggests that ongoing; alpha(1) synthesis is low. Based on the detection of alpha(1) mRNA in neonatal lens fibers, Na,K-ATPase synthesis by lens fibers may be higher during cell elongation and growth.