Sodium activity in the aqueous humor and corneal stroma of the rabbit

Molecular mechanisms of water transport in the eye

The four major sites for ocular water transport, the corneal epithelium and endothelium, the ciliary epithelium, and the retinal pigment epithelium, are reviewed. The cornea has an inherent tendency to swell, which is counteracted by its two surface cell layers, the corneal epithelium and endothelium. The bilayered ciliary epithelium secretes the aqueous humor into the posterior chamber, and the retinal pigment epithelium transports water from the retinal to the choroidal site. For each epithelium, ion transport mechanisms are associated with fluid transport, but the exact molecular coupling sites between ion and water transport remain undefined. In the retinal pigment epithelium, a H+-lactate cotransporter transports water. This protein could be the site of coupling between salt and water in this epithelium. The distribution of aquaporins does not suggest a role for these proteins in a general model for water transport in ocular epithelia. Some water-transporting membranes contain aquaporins, others do not. The ultrastructure is also variable among the cell layers and cannot be fitted into a general model. On the other hand, the direction of cotransport in symporters complies with the direction of fluid transport in both the corneal epi- and endothelium, as well as the ciliary epithelium and retinal pigment epithelium.

The resiliency of the corneal endothelium to refractive and intraocular surgery

PURPOSE

To describe stress factors (phenylephrine and contact lenses) from the corneal epithelium that can affect the corneal endothelium, and to describe the effects of refractive and intraocular surgery on the corneal endothelial structure and function.

METHODS

Significant clinical and experimental publications are reviewed and recent experiments conducted in the author's laboratory to describe the corneal endothelial stresses.

RESULTS

The corneal epithelium serves as a barrier to topical phenylephrine (2.5-10%). In a compromised epithelium, topical phenylephrine will cause drug-induced stromal edema and endothelial vacuolization. Contact lenses are capable of stimulating the epithelial arachidonic acid cascade to release 12(R)hydroxyeicosatetraenoic acid (12(R)HETE) and 8(R)hydroxy-hexadecatrienoic acid (8(R)HHDTrE) to cause endothelial Na+/K+ adenosine triphosphatase (ATPase)-inhibition and polymegethism. Specular microscopy of the corneal endothelial cells after refractive surgery (photorefractive keratectomy [PRK], laser in situ keratomileusis [LASIK], intrastromal rings [INTACs]) has shown that there is minimal effect. However, laser ablation of the stroma within 200 microm of the corneal endothelium will result in endothelial cell structural changes and the formation of the amorphous substance deposited onto Descemet's membrane. Phacoemulsification with a high flow of the irrigation solution can alter the endothelial surface glycoprotein layer. Lidocaine hydrochloride (1%) used as intracameral anesthesia readily diffuses through the corneal endothelium, resulting in stromal uptake and endothelial cell swelling. With phacoemulsification, however, the washout of lidocaine from the cornea (T1/2, 5 minutes) and iris (T1/2, 9 minutes) occurs quickly. Corneal endothelial wound healing after keratoplasty occurs in the following sequence: migration of endothelial cells, development of tight junctions, and the formation of Na+/K+ ATPase pump sites.

CONCLUSIONS

Corneal endothelial resiliency is due to the increased peripheral endothelial cell number for migration, the ability of endothelial cells to form tight junctions to maintain the endothelial barrier, the increase in endothelial Na+/K+ ATPase pump sites under stress, and the ability of the corneal endothelial cells to shift their metabolism of glucose to the hexose monophosphate shunt for the production of nicotinamide adenine dinucleotide phosphate (NADPH) and membrane repair. All of these factors are important, along with the aqueous humor sodium concentration, which establishes the osmotic gradient for corneal deturgescence and transparency.

Comparison of rabbit and human corneas stored in Optisol-GS: changes in stromal sodium

PURPOSE

To evaluate the hydration, and the levels of free, total and bound sodium in fresh rabbit corneal stromas and also those preserved for up to 21 days in Optisol-GS. The effect of epithelial removal on stromal sodium and hydration parameters was also evaluated. Trends in stromal hydration and sodium environment were compared to results we previously obtained using human eyes stored under identical conditions.

METHODS

Stromal hydration was evaluated thermogravimetrically. A sodium-specific electrode and an atomic absorption spectrophotometer were used to determine the amounts of free and total stromal sodium, respectively. In one cornea of each pair, the epithelium was removed prior to placement in the storage media. After 3, 7, 14 or 21 days at 4 degrees C, corneas were removed from the Optisol-GS, at which time sodium and hydration measurements were obtained.

RESULTS

With an intact epithelium, the hydration of the rabbit stromas was elevated significantly at each day of storage compared to fresh corneas. Free and total sodium levels of rabbit stromas did not differ statistically from fresh values, however the bound sodium values did increase during storage. In the absence of the epithelium, the stromal hydration and sodium content (free, total and bound) were significantly elevated and the increase was much greater than in corneas stored with an intact epithelium. These findings differ from those we measured previously using human tissue.

CONCLUSIONS

Rabbit corneas responded differently from human corneas to storage in Optisol-GS. The hydration levels increased to a greater level in rabbit than human corneas under both storage conditions. The trends in amounts of both free and total sodium were similar between the species, although the absolute amounts differed. The largest discrepancy was observed in the amount of bound sodium, with the rabbit corneas experiencing large increases not documented in the human tissue. These results suggest that direct comparisons of stromal hydration and ionic environment between the species should be approached with caution.

Fluid and ion transport in corneal endothelium: insensitivity to modulators of Na(+)-K(+)-2Cl- cotransport

The role of Na(+)-K(+)-2Cl- cotransport in ion and fluid transport of the corneal endothelium was examined by measuring changes in corneal hydration and uptake of 86Rb by the endothelial cell layer. Isolated, intact rabbit corneas maintain normal hydration when they are superfused at the endothelial surface with bicarbonate (HCO3-)-Ringer solutions as a result of equilibrium between active ion and fluid transport out of the stromal tissue and leak of fluid into stromal tissue from the aqueous humor. Furosemide and bumetanide did not alter this equilibrium when they were added to the superfusion medium. Uptake of 86Rb by the endothelium of the incubated cornea was increased 25% by bumetanide, but uptake in the presence of ouabain (70% less than that of controls) was not changed by bumetanide. In Na(+)-free medium, uptake of 86Rb was reduced by 58%, but it was unchanged in Cl(-)-free medium. Calyculin A, a protein phosphatase inhibitor and activator of Na(+)-K(+)-Cl- cotransport, was without effect on 86Rb uptake. Hypertonicity (345 mosmol/kg) increased uptake slightly, whereas hypotonicity (226 mosmol/kg) caused a 33% decrease. Neither of these changes was significantly different when bumetanide was present in the media. It is concluded that Na(+)-K(+)-2Cl- cotransporter activity is not exhibited by the in situ corneal endothelium and does not play a role in the ion and fluid transport of this cell layer. Its presence in cultured endothelial cells may reflect the reported importance of this protein in growth, proliferation, and differentiation.