Renal inner medullary sorbitol metabolism

Sorbitol participates in the osmoregulation of several renal cells and has also been found in isolated inner medullary collecting duct (IMCD) cells in primary culture. Therefore, osmotic regulation and distribution of sorbitol and the key enzymes of sorbitol metabolism, aldose reductase and sorbitol dehydrogenase in the renal inner medulla, were investigated in vivo under various osmotic conditions (control, diuresis, antidiuresis). In homogenates of the renal inner medulla of Wistar rats, the sorbitol content correlated with the urine osmolarity [68 +/- 12 mumol/g protein (control), 28 +/- 9 mumol/g (diuresis), 110 +/- 15 mumol/g (antidiuresis)]. Similar results were obtained for the activity of aldose reductase (sorbitol synthesis) [25 +/- 4 U/g (control), 19 +/- 3 U/g (diuresis), and 48 +/- 7 U/g (antidiuresis)]. On the contrary, the activity of sorbitol dehydrogenase (sorbitol degradation) was significantly increased to 1.26 +/- 0.42 U/g under diuretic conditions vs. control (0.84 +/- 0.14 U/g, P < 0.05). These results demonstrate the correlation between the enzymes of sorbitol synthesis and sorbitol degradation in the intact inner medulla and the urine osmolarity in vivo. Whereas the aldose reductase activity was 2.3-fold enriched in IMCD cells, the specific activity of sorbitol dehydrogenase was relatively increased in a preparation of enriched interstitial cells. This distribution was not dependent on the various diuretic conditions. These results indicate that enzymes of synthesis and of degradation of sorbitol are osmotically regulated in vivo. Therefore, the enzymatic activities of sorbitol synthesis appear to be primarily located in epithelial cells, whereas enzymatic activities of sorbitol degradation seem to be localized in interstitial cells of the renal inner medulla.