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 capac ity. The authors tested whether different Na,K-ATPase polypeptides are expressed in the two cell types and whether both cells have the machi nery needed for ongoing Na,Ii-ATPase expression as judged by the prese nce of mRNA for the Na,K-ATPase alpha subunit. Methods. Membranes were isolated from adult rat lens epithelium or fibers, and Western blot e xperiments were conducted for Na,K-ATPase alpha(1), alpha(2), and alph a(3) polypeptides. Total RNA was isolated from adult rat lens epitheli um or fiber cells, and Northern analysis was conducted for Na,K-ATPase alpha(1), alpha(2), and alpha(3) mRNA. Some experiments were conducte d using fiber cells from neonatal (3-day-old) rat lenses. Results. Mul tiple isoforms of Na,K-ATPase were detected in adult rat lens epitheli um. 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 o bserved as sharp bands at 100 to 108 kDa. In fiber cells, only Na,R-AT Pase alpha(1) immunoreactive polypeptide was detected. Judged by immun oblot 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 analysi s was conducted with RNA from neonatal (3-day-old) lens fibers; Na,K-A TPase alpha(1) mRNA was clearly visible. Conclusions. Adult rat lens e pithelium expresses more than one isoform of Na,K-ATPase catalytic sub unit, whereas only the alpha(1) isoform can be detected in fiber cells . In adult rat lens fiber cells, the observation of alpha(1) polypepti de, 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 elonga tion and growth.