Effects of ascorbic acid, aminoguanidine, sorbinil and zopolrestat on sorbitol and betaine contents in cultured rat renal cells

Abstract

Aldose reductase inhibitors (ARI) have been developed to reduce the conversion of high glucose levels to sorbitol, an important renal osmolyte that may rise to damaging levels in many tissues during diabetic hyperglycemia. Ascorbic acid (AA) and aminoguanidine (AMG) have also been reported to reduce sorbitol levels in diabetes: AMG in rat kidney, and AA in guinea pig lens and human erythrocytes. We tested the effects of AMG, AA, and Pfizer’s sorbinil and zopolrestat for 48 h on primary rat renal cell cultures, established from renal inner medullas of male Wistar rats 8–12 weeks old. Osmolyte contents in scraped cells were analysed by HPLC: 100 µM sorbinil and 20 µM zopolrestat decreased sorbitol levels (P<0.05 and P<0.001, respectively), and increased the content of another osmolyte, betaine (P<0.01 and <0.01, respectively). The quantity of ATP in cells was unchanged, suggesting no short-term problems. In contrast, 10 mM AMG and 10 mM AA had no effect on sorbitol contents (in contrast to some previous studies). We then tested aldose reductase (AR) activity in crude homogenates of rat lens and renal inner medulla, with glyceraldehyde substrate. For both tissues, 5 µM zopolrestat inhibited AR activity by 92–94% (P<0.002); 10 mM AA by 16–20% (P<0.02); and 10 mM aminoguanidine by 22–24% (P<0.03). We conclude that AMG and AA are not readily usable as inhibitors of renal AR.