NA,K-ATPASE POLYPEPTIDE UP-REGULATION RESPONSES IN LENS EPITHELIUM

PURPOSE. In a previous study, an increase in Na,K-ATPase alpha(2) expr ession was detected in the epithelium of porcine lenses exposed to amp hotericin B, an ionophore that also increases lens sodium and stimulat es active sodium transport. The purpose of the present study was to de termine whether an increase of Na,K-ATPase alpha(2) synthesis is a res ponse to an episode of rapid Na-K transport or whether the increase in lens sodium alone can initiate the response. METHODS. Western blot an alyses were conducted to probe for Na,K-ATPase alpha polypeptides in m embrane material isolated from porcine lens epithelium. Ouabain-sensit ive adenosine triphosphate hydrolysis was used as an index of Na,K-ATP ase activity, and lens ion content was determined by atomic absorption spectrophotometry. 86-Rubidium (Rb-86) uptake was measured as an indi cator for active potassium transport. RESULTS. Rb-86 uptake was marked ly diminished in lenses exposed to dihydro-ouabain (DHO), signifying i nhibition of active sodium-potassium transport. Consistent with this, the sodium content of DHO-treated lenses increased. By western blot an alysis, a marked increase of Na,K-ATPase alpha(2) polypeptide could be detected in the epithelium of DHO-treated lenses. To rule out the pos sibility that apparent stimulation of Na,K-ATPase alpha(2) synthesis s temmed from binding of DHO to Na,K-ATPase sites, experiments were cond ucted to confirm an increase of Na,K-ATPase alpha(2) polypeptide in th e epithelium of lenses exposed to low-potassium medium to inhibit acti ve sodium-potassium transport. Consistent with the apparent increase o f Na,K-ATPase polypeptide, Na,K-ATPase activity was detectably increas ed in epithelial material isolated from lenses pretreated with DHO or low-potassium medium. CONCLUSIONS. An increase in Na,K-ATPase alpha(2) polypeptide call occur in the epithelium of lenses subjected to an ep isode of sodium pump inhibition. This suggests the response could be t riggered by an increase in cell sodium and does not necessarily requir e a period of stimulated active sodium-potassium transport.