Aqueous/vitreous tonicity in "sugar' cataracts.

We examined the aqueous/vitreous tonicity in "sugar' cataracts. Four sets of eyes and their controls were examined: diabetic human eyes, eyes from rabbits and rats fed 50% galactose for up to 1 week, and eyes from streptozotocin-diabetic rats of up to 21 days after diabetes induction. The results showed no statistically significant increase of aqueous/vitreous tonicity in the diabetic human eye and eyes from galactose-fed rats and rabbits. Diabetic rats, on the other hand, showed large tonicity increase in the vitreous, although treatment of the diabetic rats with an aldose reductase inhibitor or insulin completely normalized the tonicity. Seven days after feeding, the galactose-fed animals already showed lens opacities, while diabetic rat lenses remained clear even after 21 days after diabetes induction. These results indicate insignificant osmotic changes in the human eye. In the rat eye, the increase of lens tonicity can be offset by a concomitant increase in the vitreous tonicity. However, this phenomenon is not observed in the galactose eyes. The nature of the osmolytes in the diabetic eye is unclear, although both glucose and sorbitol have been ruled out. The presence of an osmotic regulatory mechanism in the eye is implied.