Influence of amphotericin B on the sodium pump of porcine lens epithelium

Active transport by Na⁺-K⁺-ATPase in the monolayer of lens epithelium is vital for the regulation of sodium and potassium levels within the mass of fiber cells that make up the bulk of the lens. In this study, experiments were conducted using porcine lenses to test whether Na⁺-K⁺-ATPase activity in the epithelium is altered when the permeability of lens cell plasma membranes is increased by the ionophore amphotericin B. After 24 h, sodium was significantly (P < 0.01) elevated in lenses exposed to 5 or 10 μM amphotericin B. Amphotericin B stimulated ⁸⁶Rb uptake, probably through an increase of cytoplasmic sodium concentration due to increased inward sodium leak; the rate of ouabain-sensitive potassium (⁸⁶Rb) uptake by intact lenses was significantly increased by amphotericin B at 5 μM (P < 0.05) and 10 μM (P < 0.01). After 24 h, the epithelium from lenses exposed to amphotericin B had an Na⁺-K⁺-ATPase activity that was more than twofold higher (P < 0.01) than the Na⁺-K⁺-ATPase activity in control lenses. By immunoblot, there was an increase in Na⁺-K⁺-ATPase catalytic (α) subunit immunoreactive polypeptide in the epithelium of lenses exposed to amphotericin B. The increase stemmed from a marked increase of Na⁺-K⁺- ATPase α₂-immunoreactive polypeptide but little change in the amount of α₁-immunoreactive protein. As judged by immunoblot experiments, the amount of Na⁺-K⁺-ATPase β₁-immunoreactive polypeptide also appeared to be higher in the epithelium of amphotericin B-treated lenses compared with control lenses. In summary, these results suggest that in response to a permeability challenge with amphotericin B, the porcine lens epithelium is able to increase the activity of Na⁺-K⁺-ATPase. The same permeability challenge also appears to stimulate the biosynthesis of Na⁺-K⁺-ATPase catalytic subunit as well as glycoprotein subunit polypeptides.