Sorbitol Metabolism in the Retina: Accumulation of Pathway Intermediates in Streptozotocin Induced Diabetes in the Rat

Abstract

Summary. The capacities of rat lens and retina to accumulate sorbitol pathway intemiediates in streptozotocin induced diabetes were inve.stigated. The experiments performed were of two types:(i) investigations of the time course of the appearance of the pathway intermediates in lens and retina following the induction of diabetes, and (ii) investigation of the relationship of scirbitol pathway intermediate accumulations to varying doses of .strt'ptozotocin. The experimental objectives were to investigate the factors which control the activity of the pathway in vivo, and to a.ssfss the proposed relation.ship between sorbitol metabolism in the retina and the pathogenesis of diabetic small vessel disease in this tissue.The sorbitol which accumulated in the retina of the diabetic animal (approximately 1-5 iimolc/g protein) was formed by the intrinsic metabolic activity of the tissne and was not attributed to transport from an external metabolic site. The accunmlation of this intermediate was directly related to the plasma gincose concentration and could not be related to other parameters of the diabetic state, e.g. plasma free fatty acid concentration, ketonaemia.It was confirmed that the rat lens accumulated sorbitol (approximately 80 (imole/g protein) and fructose (approximately 20 n'Tiole/g protein) in diabetes, and that such accumulations were related to the glucose concentration of the aqueous humour. The possibility that factors associated with kctoacidosis aflected the relati\e concentration.s of lens sorbitol and fructose was suggested by the results.An hypothesis which invoked sorbitol iiccuninlation to osmotically signifieant concentrations within the retina as a pathogenic agent in diabetic small ve.ssel disease in this tissue conld not be directly supported by the results. Alternate mechanisms by which low concentrations of sorbitol iu diabetic retina could act as a pathogenic stimulus are discussed.

The Localisation of Sorbitol Pathway Activity in the Rat Renal Cortex and Its Relationship to the Pathogenesis of the Renal Complications of Diabetes Mellitus

“…The conclusion that significant sorbitol pathway activity is absent in the renal glomerulus was supported by the finding that glomeruli incubated in vitro under a high glucose concentration accumulated glucose to high intracellular concentrations. It has been demonstrated that, in those tissues in which the sorbitol pathway is active, accumulation of the pathway intermediates accompanies conditions under which the intracellular glucose concentration is increased (Winegrad, Clements and Morrison, 1972;Hutton et al, 1974). That the glomeruli incubated in vitro maintained metabolic activity was attested by the finding that they incorporated radioactivity from [U-"C] glucose into major metabolic end products.…”

“…A series of incubations was performed which was identical to the above, with the exception that radioisotope was deleted from the media, and that at the end of the incubation period (2-5 h) the glomeruli were isolated by centrifugation (2 min at 1,000 g), washed in ice-cold 0-9% (w/v) NaCl and then frozen in liquid nitrogen. The concentrations of glucose, fructose, sorbitol and myo-inositol in the frozen glomeruli were then determined by the previously described method (Hutton et al, 1974).…”

“…The concentrations of glucose, fructose, sorbitol and myo-inositol in the frozen glomeruli were then determined by the previously described method (Hutton et al, 1974).…”

“…Beaumont et al (1971) have proposed a similar mechanistic basis for the pathogenesis of diabetic small vessel disease of the retina. The concentration of sorbitol which accumulates in the retina in the diabetic state is, however, much lower than that which accumulates in the lens and would need to be confined to a small cellular compartment in the retina if it were to result in osmotic damage to the tissue (Hutton et al, 1974). The morphological and functional lesions characteristic of diabetic small vessel disease in the retina are found in many other tissues of the human diabetic (Bloodworth, 1963;Ditzel, 1968) and it is thus not improbable that such lesions have a common pathogenic basis.…”

Polyol Metabolism in Monkey‐Kidney Epithelial‐Cell Cultures

“…After removal of the vessel and contents from the liquid nitrogen, a suspension of the cells in the vessel was obtained by adding 5 ml ice-cold water to the vessel, and briefly shaking at room temperature. The suspension was then removed from the vessel, and sonified for 15 s at 0°C (Branson sonifier B-12; microtip setting 6), and samples were then removed for the determination of protein [12] and for the gas chromatographic analysis of the intracellular free carbohydrates as their trimethylsilyl derivatives [13]. As a confirmatory analysis, sorbitol in the sonified cellular material was assayed enzymatically by the method of Williams-Ashman [14].…”