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Laboratory Science |

Storage of Human Corneas in Dextran and Chondroitin Sulfate–Based Corneal Storage Medium:  Changes in Stromal Free Sodium

Monica M. Jablonski-Stiemke, PhD; Henry F. Edelhauser, PhD
Arch Ophthalmol. 1998;116(5):627-632. doi:10.1001/archopht.116.5.627.
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Objectives  To evaluate the hydration and the levels of free and total sodium in human corneal stromata preserved for up to 21 days in a dextran and chondroitin sulfate–based corneal storage medium (Optisol-GS, Chiron IntraOptics, Irvine, Calif) and to evaluate the effect of epithelial removal on stromal sodium and hydration parameters.

Methods  Stromal hydration was evaluated thermogravimetrically. A sodium-specific electrode and an atomic absorption spectrophotometer were used to determine the amounts of free and of total stromal sodium, respectively, of preserved human corneas. In 50% of the corneas, the epithelium was removed prior to placement in the storage medium. After 3, 7, 14, or 21 days at 4°C, corneas were removed from the storage medium and sodium measurements were taken.

Results  In corneas with an intact epithelium, the stromal hydration as well as the stromal free sodium and total sodium levels were relatively constant up to 21 days of storage in the preservation medium. In the absence of the epithelium, the water and sodium contents of the stroma increased significantly during storage.

Conclusion  The presence of an intact epithelium is required for maintaining the hydration and sodium levels within the corneal stroma during storage. Removal of the epithelium prior to storage results in increased sodium values and hydration, which may affect postkeratoplasty deturgescence.

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Figure 1.

Illustration of the in vivo transendothelial gradient for sodium (Na+) for a rabbit cornea. A, The free sodium (sodium activity) of the aqueous humor is greater than that of the stroma. This gradient in free sodium generates an osmotic force of 98.5 mm Hg in response to which water moves out of the stroma, accounting for stromal deturgescence. B, Detail of boxed area in A showing that the total concentration of sodium within the stroma is divided into 2 components, free and bound. The net negative charge of the stromal proteoglycans attracts ionically extracellular cations, rendering some sodium molecules electrochemically inert. These bound ions are unable to participate in electrochemical reactions. The remainder of the sodium is free and able to generate osmotic forces that are proportional to the difference in activity across a semipermeable membrane, as shown in A.

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Figure 2.

Stromal hydration of human corneas stored in a dextran and chondroitin sulfate–based preservation medium (Optisol-GS, Chiron IntraOptics, Irvine, Calif) at 4°C. Percent values are expressed as mean ± SEM.

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Figure 3.

A, Free sodium (Na+) values, B, total sodium values, and C, bound sodium values for human stromata stored in a dextran and chondroitin sulfate–based preservation medium (Optisol-GS, Chiron IntraOptics, Irvine, Calif) at 4°C. Milliequivalents per liter values are expressed as mean ± SEM.

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Figure 4.

Illustrative interpretation of the fluxes of sodium (Na+), water, and colloids across the corneal barriers during storage in a dextran and chondroitin sulfate–based preservation medium (Optisol-GS, Chiron IntraOptics, Irvine, Calif). A, The epithelium serves as a barrier to limit the diffusion of dextran, chondroitin sulfate, and sodium into the corneal stroma. These same molecules can then osmotically keep water from diffusing into the stroma. Stromal proteoglycan loss is also minimized. By limiting the diffusion of these substances, the number of free and total sodium molecules within the stroma remains constant. There is minimal movement of larger molecules across the endothelial surface. B, Without the epithelium, dextran, chondroitin sulfate, sodium, and water diffuse into the stroma because the epithelial barrier is no longer present. Stromal proteoglycan loss is increased. A combination of all these factors leads to an increase in the number of total, free, and bound sodium molecules within the stroma by diffusion from the storage medium. There is minimal diffusion of larger molecules across the endothelial surface.

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