Postischemic proximal tubular resistance to oxidant stress and Ca2+ ionophore-induced attack. Implications for reperfusion injury.

BACKGROUND

The severity of "reperfusion injury" is dependent on the extent to which the involved pathways are activated and on the degree of tissue susceptibility to them. This study was undertaken to ascertain whether preexistent ischemic proximal tubular damage (ischemic "pre-conditioning") significantly alters the expression of two purported mediators of reperfusion damage: oxidant stress and cytosolic Ca2+ loading.

EXPERIMENTAL DESIGN

Male Sprague-Dawley rats underwent 35 minutes of bilateral renal arterial occlusion. Fifteen minutes or 24 hours later, the kidneys were removed, proximal tubular segments (PTS) were isolated, and their susceptibility to oxidant stress (H2O2 or FeSO4) and to cytosolic Ca2+ loading (Ca2+ ionophore, A23187) was determined. Results were contrasted to those obtained with normal PTS. Cell injury was quantified by percentage of cellular lactate dehydrogenase released. Lipid peroxidation was gauged by PTS malondialdehyde (MDA) concentrations. As an index of endogenous antioxidant defenses, PTS catalase and superoxide dismutase activities were determined. Vulnerability to lipid peroxidation is highly dependent on phospholipid unsaturated fatty content, so PTS fatty acid concentrations also were assessed.

RESULTS

Although PTS harvested at 15 minutes postischemia manifested sublethal injury (increased lactate dehydrogenase release under control conditions), no increased vulnerability to the oxidant insults or to the Ca2+ ionophore was noted. By 24 hours of reflow, cytoresistance to each of the insults had developed. Postischemic PTS demonstrated no increase in basal MDA concentrations (indicating a lack of in vivo lipid peroxidation), and when challenged with H2O2 or FeSO4, significantly less MDA generation developed (vs. the normal PTS). This resistance to lipid peroxidation was not associated with increased superoxide dismutase/catalase levels or altered PTS fatty acid content.

CONCLUSIONS

Sublethal ischemic proximal tubular injury does not directly predispose to oxidant stress or cytosolic Ca2+ loading, and by 24 hours postischemia, increased resistance to these insults develops. Decreased membrane susceptibility to lipid peroxidation may contribute to this result.