Corneal tolerance of vitrifiable concentrations of propane-1,2-diol

The merit of corneal cryopreservation by vitrification as opposed to conventional freezing is the avoidance of ice damage which is believed to disrupt the integrity of the corneal endothelium resulting in loss of corneal transparency. The cornea must be equilibrated with high concentrations of cryoprotectant in order to achieve vitrification at practicable cooling rates. In an earlier study, corneas were exposed to 3.4 mol/liter propane-1,2-diol (Rich and Armitage (1990) Cryobiology 27, 42-54). The present study exposed rabbit corneas to concentrations of propane-1,2-diol between 3.4 and 5.4 mol/liter in a Hepes-buffered Ringer's solution containing glutathione, adenosine, 5 mmol/liter sodium bicarbonate, 6% (w/v) bovine serum albumin, and 2.5% (w/v) dextran sulfate. Dextran sulfate was as effective as chondroitin sulfate at improving endothelial tolerance of 3.4 mol/liter propane-1,2-diol. This beneficial effect may be linked to the polyanionic nature of these molecules. Corneas exposed to 5.4 mol/liter propane-1,2-diol were cooled in liquid nitrogen vapor at a temperature of -140 degrees C for 2 h. Warming was achieved by direct transfer to a dilution solution at -10 degrees C. Endothelial function was assessed by monitoring corneal thickness during perfusion of the endothelial surface at 34 degrees C for 6 h. Endothelial structure was observed by specular microscopy during the perfusion and by scanning electron microscopy after perfusion. Corneas tolerated exposure to 3.4 mol/liter propane-1,2-diol for 20 min at 0 degrees C and to 4.1 mol/liter for 10 min at -10 degrees C. Exposure to 4.8 and 5.4 mol/liter for 10 min at -10 degrees C caused endothelial damage, although a degree of endothelial function was retained. Function following exposure to 5.4 mol/liter was improved by reducing the temperature of exposure to -15 degrees C. Corneas cooled after exposure to 5.4 mol/liter propane-1,2-diol for 10 min at -15 degrees C apparently vitrified, but devitrified on warming. The corneas swelled to such an extent during perfusion that the endothelium could not be viewed by specular microscopy, subsequent scanning electron microscopy showed a severely disrupted endothelium.