DECREASED EXPRESSION OF MITOCHONDRIAL-DERIVED H2O2 AND HYDROXYL RADICAL IN CYTORESISTANT PROXIMAL TUBULES

Increased production of reactive oxygen metabolites (ROM) can contribu te to the initiation phase of nephrotoxic and ischemic acute renal fai lure (ARF). However, whether altered ROM expression also exists during the maintenance phase of ARF has not been adequately assessed. Since diverse forms of tubular injury can initiate a ''cytoresistant state,' ' this study tested whether a down-regulation of ROM expression might develop in the aftermath of acute tubular damage, potentially limiting renal susceptibility to further attack. To test this hypothesis, rats were subjected to either mild myohemoglobinuria (glycerol injection) or bilateral ureteral obstruction and 24 hours later, cytoresistant pr oximal tubular segments (PTS) were isolated to assess ROM expression. PTS from sham operated rats were used to establish normal values. Both sets of cytoresistant PTS manifested -75% reductions in H2O2 levels, as assessed by the phenol red/horseradish peroxidase technique (P < 0. 01 to 0.001). A 40% reduction in hydroxyl radical ((OH)-O-.) levels wa s also observed (salicylate trap method), thereby substantiating decre ased oxidant stress in cytoresistant PTS. Catalase, glutathione peroxi dase, and free iron levels were comparable in control and cytoresistan t PTS? suggesting that decreased H2O2 production (such as by mitochond ria) was the cause of the decreased oxidant stress. To test this latte r hypothesis, H2O2 expression by control and cytoresistant PTS was ass essed in the presence of respiratory chain inhibitors. Although site 1 and site 3 inhibition markedly suppressed H2O2 production in control PTS, they had no impact on H2O2 production in cytoresistant PTS, imply ing that production at these sites was already maximally suppressed. C orrelates of the decreased mitochondrial H2O2 production were improvem ents in cell energetics (increased ATP/ADP ratios with Na ionophore tr eatment) and similar to 40 to 90% increases in PTS/renal cortical glut athione content. We conclude that: (I) proximal tubule H2O2/(OH)-O-. e xpression can be downregulated during the maintenance phase of ARF; (2 ) this seemingly reflects a decrease in mitochondrial ROM generation; and (3) the associated improvements in glutathione content and/or cell ular energetics could conceivably contribute to a post-injury cytoresi stant state.