(Na+K+)-activated ATPase in human cornea. Distribution within the cornea and properties of the enzyme from epithelial cells

Corneal Protein Nitration in Experimental Uveitis

Increased expression of inducible nitric oxide synthase (NOS-2) in inflammatory diseases like uveitis suggests that it contributes to the observed pathological state. The aim of this study was to evaluate corneal expression of NOS-2 and corneal protein nitration in a rat model of uveitis. A single injection of intravitreal lipopolysaccharide was used to induce uveitis. Corneal proteins were separated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and visualized by Coomassie blue staining. Expression of NOS-2 and nitrotyrosine (NO2Tyr) formation were determined via immunohistochemistry and Western blot analysis. Total nitrate/nitrite levels in the vitreous were measured by spectral analysis via the Griess reagent. Immunohistochemical analysis revealed increased corneal NOS-2 and NO2Tyr immunoreactivity in rats with uveitis compared with controls. NOS-2 and NO2Tyr immunoreactivity was observed in and around basal cells in the corneal epithelium. Western blot analysis of corneal lysates showed multiple nitrated protein bands in uveitic rats. Spectrophotometric measurement of total nitrate/nitrite levels in the vitreous affirmed significantly increased levels of nitric oxide generation in uveitis (126 ±2.63 μ M/mg protein) compared with controls (65 ±6.57 μ M/mg protein). The presented data suggests that extensive formation of protein nitration and reactive nitrogen species in the cornea contributes to tissue destruction in uveitis. Hence, selective inhibition of NOS-2 may prevent long-term complications and lead to an improvement in the management of uveitis.

Specific simple sugars promote chemotaxis and chemokinesis of corneal endothelial cells

Bovine corneal endothelial cells showed a strong migratory response to specific simple sugars (D-glucose and sucrose, but not L-glucose, sorbitol, lactose, or D-galactose) at concentrations above 10 mM. Checkerboard analysis of the migratory responses in modified Boyden chambers indicated both chemotactic and chemokinetic effects. Serum starvation of the cultures increased the chemotaxis towards D-glucose and 2-deoxy-D-glucose, but not towards sucrose. Migration to sucrose and glucose was inhibited by chelation of extracellular calcium or by inhibition of Na+, K+ ATPase with ouabain. To date, this migratory response has been found only in corneal endothelial cells. Neither human melanoma cells, human breast carcinoma cells, bovine aortic endothelial cells, nor bovine microvascular endothelial cells migrated towards simple sugars, although all cell types migrated toward fibronectin in chemotaxis assays. After 16-19 passages in culture, bovine corneal endothelial cells retained their ability to migrate towards fibronectin, but lost their ability to migrate towards sugars. This loss of migratory response was accompanied by a sevenfold decrease in Na+, K+ ATPase activity. Although loss of Na+, K+ ATPase activity accompanied the loss of migratory response, pretreatment of cell cultures with 25 mM glucose did not stimulate, but rather lowered Na+, K+ ATPase activity in low or high passage cultures.

Gap junctions in rabbit corneal epithelium: limited permeability and inhibition by cAMP

Rabbit corneas were mounted in Ussing chambers, and the apical membrane of the superficial cells (SCs) was permeabilized by exposure to digitonin in a Ca(2+)-free Ringer solution. This treatment resulted in the generation of large (60.7 +/- 13.2 microA/cm2, n = 25) Na(+)-dependent tear (T)-to-stroma (S) short-circuit currents (Isc). The Isc was abolished by ouabain and by 1.4 mM Ca2+ and was inhibited by heptanol, 18 alpha-glycyrrhetinic acid, and dieldrin, effects consistent with the notion that corneal transepithelial fluxes include translocations through gap junctions (GJs) before basolateral membrane transport. T-to-S Isc were also generated when T-side Na+ was replaced by K+, eliciting a T-to-S K+ flux via basolateral K+ channels and when, with either Na+ or K+ on the T side, channels were introduced at the apical membrane with amphotericin B. The Isc in all four conditions exhibited similar sensitivity to GJ inhibitors and were inhibited by adenosine 3',5'-cyclic monophosphate (cAMP) elevation. Fluorophotometry combined with SC permeabilization with digitonin demonstrated that the half-time for the SC to sub-SC movement of 2',7'-bis(carboxyethyl)-5,6-carboxyfluorescein (mol wt 540) exceeded 3 h. These results indicate that junctional communications along the epithelial stratification axis are highly restricted and modulated by cAMP concentration.

Visualization of ouabain binding in bovine cornea

In these experiments, the binding of ouabain, a specific inhibitor of NA,K-ATPase is visualized in bovine cornea using an immunohistochemical method. Due to cross-reactivity between ouabain and digitoxin antiserum, the ouabain binding is demonstrated after an immunological labelling with fluorescein. As shown in the corneal tissue sections and in the epithelial and endothelial cell preparations, a bright fluorescence of the endothelial cell membranes is observed in corneae incubated in 10(-5)-10(-3) mol/l ouabain. In the epithelium, only a weak binding of the ouabain is observed at the basolateral membrane after incubation in 10(-3) mol/l ouabain.

Sodium and potassium saturation kinetics of Na+K+-ATPase in plasma membranes from corneal endothelium: fresh tissue vs. tissue culture

The maximal velocity (Vmax) and the apparent dissociation constant (K0.5) of Na+K+-ATPase have each been estimated with respect to sodium and potassium ion activation. These estimations were made from the enzymatic activity of plasma membrane preparations derived from bovine corneal endothelial cells. The determinations were made on cells obtained from fresh tissue and from secondary tissue cultures. Two methods were used to obtain the estimates: the first used a combination of Eadie-Hofstee and Hill plots; the second used Eisenthal-Cornish-Bowden plots. The Vmax for sodium was 5.58-5.60 mumol Pi/mg protein/30 min for fresh tissue vs. 2.00-1.80 mumoles Pi/mg protein/30 min for tissue cultures. The corresponding K0.5 values were 62-57 mM (fresh tissue) vs. 7.9-6.7 mM (tissue culture). Vmax for potassium was 4.28-4.00 mumoles Pi/mg protein/30 min for fresh tissue vs. 1.37-1.34 mumoles Pi/mg protein/30 min for tissue cultures. The corresponding K0.5 values were 3.3-3.1 mM (fresh tissue) and 1.7-1.7 mM (tissue culture). The results indicate a lowered activity and change in affinity of the enzyme for the two ions in tissue cultures compared to fresh tissues. The detergent Lubrol W-X increased activity in both tissue sources. Sonication had no significant effect on the activity. Variations in pH (7-9) indicated that the highest activity was obtained at pH 7.8 for the enzyme in tissue culture while activity was highest at pH 8.0 for the enzyme in fresh tissues. These differences in kinetic activity suggest a response to changes in the ion requirements of these cells due to their environment, developmental stage or some other parameter.

Anion sensitive ATPase in human cornea

In human cornea an anion sensitive ATPase is present. The highest specific activity was found in the endothelium, whereas the epithelium contained the highest total activity. The enzyme was stimulated by bicarbonate and sulfite and inhibited by thiocyanate. the majority of the enzyme was localized in the mitochondria but some activity was also detected in the plasma membranes. Atractyloside inhibited only the mitochondrial anion ATPase.

Localization of corneal superoxide dismutase by biochemical and histocytochemical techniques

Superoxide dismutase activity is found in the cornea of rats, dogs, rabbits and humans. A superoxide dismutase identical to the Cu-Zn enzyme is identified by biochemical and immunochemical methods in the corneal epithelium and endothelium in relative high abundance (90-100 U mg-1 protein). The enzyme quantities detected by radioimmune assay and bioactivity assay are similar, giving evidence that the enzyme present is biologically functional. Immunohistochemical techniques demonstrate the association of the enzyme with the cytoplasm of these layers. Soluble protein extracts of corneal stroma exhibit weak apparent superoxide dismutase activity which is found to be mainly due to copper associated with serum albumin.

The effects of ouabain on endothelial function in human and rabbit corneas

These experiments were undertaken to study the effects of ouabain on endothelial function in isolated perfused human and rabbit corneas. Both human and rabbit corneas swell at rates which are dose-dependent when perfused with ouabain (10(-8) to 10(-5) M). The human tissue, however, requires tenfold higher concentrations to achieve swelling rates comparable to rabbit. Dose-dependent ultrastructural changes are seen in ouabain-perfused endothelia progressing from moderate endothelial edema (7.5 X 10(-7) M) to marked edema with altered subcellular organelles and vacuolization (10(-5) M). Ouabain-perfused human corneas show marked endothelial edema even at the lowest concentrations of ouabain tested. Apical junctions remain intact, however, in both human and rabbit endothelia at all concentrations. These results suggest that endothelial Na/K ATPase is essential to pump function in both human and rabbit corneas.

Sodium and chloride transport in the isolated human cornea

22Na and 36Cl fluxes in isolated human cornea preparations were measured under short-circuited conditions. At pH 7.3-PH 7.6 sodium net flux was directed from aqueous humour to tear side, chloride net flux was statistically not different from zero. Alkalinization of the bathing solution to pH 8.6 stimulated unidirectional sodium and chloride fluxes from aqueous humour to tear side resulting in net sodium and chloride fluxes towards the tear side which both were statistically different from zero. Cyclic AMP (10(-3) M) was found to stimulate sodium and chloride unidirectional fluxes from aqueous humour to tear side, thus leading to significant net sodium and chloride fluxes in the same direction. Epinephrine (10(-4) M) increased the unidirectional chloride flux from aqueous humour to tear side more pronounced than in the opposite direction, producing a significant net chloride flux towards the tear side. The results are consistent with the hypothesis that the electrolyte pumps may under certain conditions contribute to the dehydration of the stroma.