Sorbitol metabolism in inner medullary collecting duct cells of diabetic rats

Abstract

Intracellular accumulation of sorbitol, generated fromd-glucose via the aldose reductase pathway, is thought to play an important role in diabetic complications such as lens cataracts and neuropathy. In order to elucidate the effect of diabetes on the renal inner medulla, another sorbitol-rich tissue, male Wistar rats were treated with a single dose of streptozotocin (60 mg/kg body weight, i.p.). Six wecks later total inner medullary tissue (IM) or isolated inner medullary collecting duct (IMCD) cells were prepared. In diabetic IM tissue, sorbitol content was 1.8-fold higher than in control IM tissue (134±17 vs. 74±22 μmol/g tissue protein). Sorbitol production in both normal and diabetic IMCD cells was strongly dependent on extracellulard-glucose concentration. In normal cells, for example, sorbitol production was 90±9 μmol sorbitol/g protein x h at 45 mMd-glucose compared to 13±1 μmol/g protein x h at 5 mM. At identicald-glucose concentrations sorbitol synthesis in diabetic IMCD cells was, however, always significantly higher than in control cells (122% of control at 15 mM and 126% of control at 45 mM). In addition, aldose reductase activity in diabetic IM was found to be augmented. The maximal velocity was 4.2 times higher (97±22 U/g protein vs. 23±7 U/g protein) while theK _m of the enzyme remained unchanged. Membrane permeability for sorbitol or the response to changes in extracellular osmolarity was not significantly different in diabetic IMCD cells and normal cells with correspondingly high intracellular sorbitol concentrations. Similarly the kinetic parameters ofd-glucose uptake were not altered by streptozotocin treatment. These results suggest that increased medullary sorbitol content in diabetic rats is a result of increased sorbitol synthesis due to a higher extracellulard-glucose concentration and augmented aldose reductase activity in face of an unaltered sorbitol permeability of the plasma membrane.