Studies on lipids and the activity of Na,K-ATPase in lens fibre cells

Na,K-ATPase was studied in the two cell types that make up the lens of the eye. Membrane material was isolated from lens fibre cells, which make up the bulk of the lens cell mass, and also from lens epithelial cells, which are present only as a monolayer on the anterior lens surface. Judged by immunoblotting, greater amounts of Na,K-ATPase α₁ and β₁ polypeptides were found in fibre cell membrane material than in epithelial cell membrane material. However, the Na,K-ATPase activity in epithelial cell membrane material was 20 times that measured in fibre cell membrane material. In ⁸⁶Rb uptake experiments with intact lenses, ouabain-inhibitable ⁸⁶Rb uptake was observed for lens epithelium but not for lens fibres. These findings are consistent with a low Na,K-ATPase activity in lens fibre cells even though these cells express a considerable amount of Na,K-ATPase α₁ and β₁ polypeptides. The lipid composition of lens fibre cell membranes causes them to be more ordered than epithelial cell membranes; this was confirmed by measurements of the infrared CH₂ symmetric stretching band frequency. Because lipid composition can influence Na,K-ATPase activity, experiments were conducted to determine whether the activity of Na,K-ATPase α₁β₁ is inhibited by lens fibre lipid. However, no significant difference in Na,K-ATPase activity was detected when Na,K-ATPase α₁β₁ was purified from rabbit kidney and then reconstituted with lipid that had been isolated from either lens epithelium or lens fibre cells. These studies indicate that lens fibre cells contain both Na,K-ATPase α₁ and β₁ polypeptides but have low Na,K-ATPase activity. However, the results do not support the notion that this is due to the lipid composition of lens fibre cell membranes.