Sorbitol pathway: presence in nerve and cord with substrate accumulation in diabetes.

Glucose, sorbitol, fructose, and inositol are present in peripheral nerve and spinal cord. Marked elevation of these substances occurs in these tissues in mildly diabetic animals. These alterations provide biochemical mechanisms which could be significant in the etiology of diabetic neuropathy.

Polyol accumulation in galactosemic and diabetic rats: control by an aldose reductase inhibitor.

An orally active inhibitor of aldose reductase, 1,3-dioxo-1H-benz[de]-isoquinoline-2(3H)acetic acid (AY-22,284), prevented cataractous changes in cultured lenses exposed to high concentrations of galactose. When given orally, AY-22,284 markedly decreased the accumulation of polyols in the lenses and sciatic nerves of galactosemic rats and rats with streptozotocin-induced diabetes. In addition, treatment of galactosemic rats with AY-22,284 effectively suppressed the formation of cataracts.

Altering the course of cataracts in diabetic rats.

A potent new aldose reductase (AR) inhibitor was effective in preventing cataractous changes in diabetic rats. Untreated diabetic rats developed early lens changes by 3 weeks and dense nuclear opacities by 6 to 9 weeks. In contrast, diabetic rats treated with the AR inhibitor showed no lens changes during the 5-month period of the experiment.

Cataractogenic effects of a boron hydride disulfide compound.

The disulfide form but not the sulfhydryl form of a boron hydride compound was found to be cataractogenic. Apparently this compound attaches to the sulfhydryl group of Na-K ATPase in the lens epithelium inactivating this crucial enzyme. The consequence is that a defect in the cation pump activity arises, leading to a rapid influx of Na ions and loss of K ions and marked increase in hydration. These changes are thought to lead to opacification.

Prevention and reversal of galactose cataract in rats with topical Sorbinil.

Rats fed a 50% galactose diet were treated topically in one eye with 1% Sorbinil . The eye treated with Sorbinil remained clear during the following 4-week period. Unexpectedly, the lens of the untreated eye also maintained transparency. Histologically both lenses remained normal. Moreover, the reduced dulcitol levels in the lenses of both eyes were identical. These findings suggest that the effect of topically administered Sorbinil in galactosemic rats was mainly systemic rather than local. Confirmation of this came from the observations that the extent of inhibition of polyol synthesis in these rats was found to be similar in the sciatic nerve, blood, and lens. A reversal of the galactose cataracts also was affected by Sorbinil eye drop treatment.

Aldose reductase in diabetic complications of the eye.

Aldose reductase (AR) appears to initiate the cataractous process in galactosemic and diabetic animals. Sugars in excess are converted to polyols by lens AR. In sugar cataracts, polyols accumulate to levels substantial enough to cause a hypertonicity leading to lens fiber swelling. All other changes appear secondary to polyol accumulation and lens swelling. The development of sugar cataracts can be duplicated in organ culture. In culture, the various changes that occur were minimized or did not occur when inhibitors of AR were included in the medium. Moreover, AR inhibitors were shown to effectively delay the onset of sugar cataract development in animals. A defect in the corneal epithelium of diabetics became apparent in vitrectomy. One manifestation of this problem was the delay in the reepithelialization of denuded corneas. In examining this problem experimentally, the epithelium was removed from the corneas of diabetic and normal rats. The regeneration of epithelium in corneas of diabetic rats required a longer period than in the normal. The possibility that AR, active in the epithelium, was involved in this phenomenon was investigated. The corneal epithelium was removed from both eyes of a diabetic rat. One eye was treated topically with the AR inhibitor CP-45,634 while the other served as control. The eye treated with CP-45,635 regenerated epithelium much more quickly than the untreated eye. Other AR inhibitors had similar beneficial effects.

Induction of the enzyme aldose reductase in a lens epithelial cell line from a transgenic mouse.

A lens epithelial cell line established from a transgenic mouse synthesizes high levels of the enzyme aldose reductase which converts sugars to polyols. This enzyme has been implicated in the formation of sugar cataracts in animals and with diabetic complications in man. The mouse aldose reductase has been characterized and by sodium dodecyl sulfate-polyacrylamide gel electrophoresis has an apparent molecular mass of 38,000, similar to the enzyme in rat and man. The cellular enzyme is inhibited by two aldose reductase inhibitors: Sorbinil (IC50 = 1.8 X 10(-7) M) and Alcon 1576 (IC50 = 7.8 X 10(-8) M). The amount and the specific activity of the aldose reductase can be further increased in the cells by raising the osmolarity of the medium to 500 mOSM. Although the amount of aldose reductase is increased approximately sevenfold under these conditions, alpha-crystallin, one of the main lens specific proteins, remained at about the same concentration. No detectable increase in sorbitol was found within the cells, in contrast to published reports on renal cells in which this polyol increases under similar hyperosmotic conditions; however, in the lens cells there was a five-fold increase in the inositol content, suggesting that this polyol rather than sorbitol may be used to compensate for some of the changes in the osmolarity. The induction of the enzyme aldose reductase without the apparent accumulation of its product suggests a complex mechanism for osmoregulation in the lens cells.

Effect of prolactin on galactose cataractogenesis.

Prolactin has been known to affect the water and electrolyte balance. Because increased lens hydration has been shown to be a common phenomenon in most, if not all types of cataracts, we have been interested in investigating a possible role of prolactin in sugar cataract induction and progression. For this study, we have used morphological and biochemical approaches. The prolactin delivery method involved intraperitoneal implantation of one or more pellets in Sprague-Dawley female rats. Following implantation of the desired number of prolactin or control (nonprolactin) pellets, animals were either fed galactose and lab chow, or lab chow diet. Gross morphological observations of whole lenses, slit-lamp examination of lenses and light microscopic analysis of lens sections showed that in the galactose-fed prolactin group, galactose associated alteration progressed faster and total opacification (mature cataract development) was achieved earlier than in the nonprolactin group. The levels of galactose and dulcitol were higher in the lenses of galactose-fed prolactin treated rats as compared to lenses from nonprolactin (control) rats. No significant difference in lens Na+-K+ ATPase activity between the prolactin and nonprolactin group was observed. Our results indicate that prolactin accelerates galactose-induced cataractogenesis in rats.

Effects of xylose on monkey lenses in organ culture: a model for study of sugar cataracts in a primate.

Lenses exposed to high concentrations of xylose in organ culture produce xylitol, and they lose transparency and exhibit other changes characteristic of cataracts. Most previous studies of this model system for cataractogenesis have employed rat or rabbit lenses, where the activity of the enzyme aldose reductase has been definitely implicated as the initiating factor. Since lenses from this species have much higher aldose reductase activity and have other differences relative to human lenses, the relevance of these findings to the human lens is uncertain. To determine the effects of xylose on the lenses of a primate, lenses from the rhesus monkey (Macaca mulatta) were incubated 24-48 hr in control medium or in TC-199 medium containing 30 mM xylose. Xylose caused a general haziness, focal swelling of epithelial cells, and swollen peripheral fiber cells, but the changes were much less pronounced than in rat lenses under similar conditions. Monkey lenses exposed to 30 mM glucose, galactose or xylose accumulated measurable sorbitol, dulcitol or xylitol, respectively, but the amounts were much lower than in rat lenses, perhaps reflecting the lower aldose reductase and higher sorbitol dehydrogenase activities in monkey lenses. The damage to monkey lenses appeared to be limited to the outer layers. In monkey lenses, xylose caused little, if any, change in membrane transport of choline or alpha-aminoisobutyrate, but severely depressed synthesis of phosphorylcholine (P-choline), and increased leakage of P-choline into the culture medium, leading to a decrease in the P-choline concentration within 24-48 hr. In summary, xylose-induced damage to monkey lenses in organ culture is qualitatively similar to that seen in rat lenses, but the changes are much less rapid and severe. Culture of monkey lenses with xylose provides a model system to extend previous studies of sugar cataractogenesis in rats to a species more closely related to humans.