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 microM 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 microM (P < 0.05) and 10 microM (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 (alpha) subunit immunoreactive polypeptide in the epithelium of lenses exposed to amphotericin B. The increase stemmed from a marked increase of Na(+)-K(+)-ATPase alpha 2-immunoreactive polypeptide but little change in the amount of alpha 1-immunoreactive protein. As judged by immunoblot experiments, the amount of Na(+)-K(+)-ATPase beta 1-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.

Differential expression of sodium pump catalytic subunits in the lens epithelium and fibers.

Na,K-ATPase in lens epithelium plays a key role in conducting sodium-potassium transport. The purpose of this study was to test whether epithelium or fiber cells can synthesize new Na,K-ATPase protein in response to an increase of membrane permeability. Western blot methodology was used to identify Na,K-ATPase alpha subunit polypeptides in membrane material isolated from lens cells. As judged by immunoblot density, epithelial cell membrane material isolated from porcine lenses cultured 24 h with 1 microM amphotericin B contained more Na,K-ATPase alpha subunit polypeptide than epithelial material isolated from control lenses. This increase stemmed from the apparent synthesis of Na,K-ATPase alpha 2 isoform polypeptide by the epithelium; Na,K-ATPase alpha 1 isoform polypeptide abundance was not detectably altered. The apparent amphotericin B-induced expression of Na,K-ATPase alpha 2 was seen in lens epithelial cells but not fiber cells. This study suggests that the epithelium of the adult porcine lens may be capable of expressing additional sodium pump molecules of the alpha 2-subtype when membrane permeability is increased.