Sodium Pump Inhibition and Regional Expression of Sodium Pump α-Isoforms in Lens

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Abstract
Both hypertension and cataract formation have been associated with reductions in sodium pump activity, possibly as a result of an endogenous inhibitor. The objective of the present study was to answer 4 closely related questions: (1) Is the lens sodium pump effectively inhibited by a labile, digitalis-like factor we have identified in the peritoneal dialysate from hypertensive patients in end-stage renal failure? (2) How does that inhibition compare to that induced by ouabain? (3) Does sodium pump isoform distribution determine the degree of lens sodium pump inhibition? (This question was precipitated by the unanticipated finding that the labile DLF was more effective in inhibiting lens sodium pump than was anticipated.) (4) Is sodium pump activity altered in lens in response to increased salt intake, a maneuver known to increase endogenous digitalis-like factor? We found that whereas ouabain produced equivalent or significantly less inhibition of lens Na(+), K(+)-ATPase from calf or rabbit, respectively, compared with brain, labile digitalis-like factor preferentially inhibited lens compared with brain. Analysis of whole-lens preparations from rabbit, calf, and normal human lens revealed substantial alpha2- and alpha3-isoforms of the sodium pump but little alpha1-isoform. Ouabain inhibition of whole-lens Na(+),K(+)-ATPase from rabbit and calf were comparable: for rabbit lens, K(i)=5.2x10(-7) mol/L; for calf lens, K(i)=1.0x10(-6) mol/L. Limited quantities of labile digitalis-like factor prohibited similar determinations; however, its concentration-activity profile paralleled that of ouabain. Na(+), K(+)-ATPase activity, measured in the 3 major anatomic regions of lens and normalized to nucleus, was greatest in epithelium (56. 9+/-17.9) compared with cortex (5.8+/-1.4) and nucleus (1.0+/-0.0; P=0.01). Immunohistochemistry of rabbit lens found abundant alpha2- and alpha3-isoforms in epithelium and limited alpha3 but undetectable alpha1 in cortex and nucleus. Finally, rats randomized to a high Na diet showed significantly reduced lens Na(+), K(+)-ATPase activity compared with those on a low Na diet, consistent with the effects of a sodium pump inhibitor. In conclusion, the present study suggests that digitalis-like factor may provide a link between hypertension and cataract formation.