Heme protein-induced tubular cytoresistance: Expression at the plasma membrane level

Abstract

Following experimental rhabdomyolysis, animals become resistant to heme protein-induced acute renal failure (ARF). The goals of this study were to: (a) ascertain whether this resistance, previously documented only in vivo, is expressed directly at the proximal tubular cell level; (b) determine whether heme proteinuria (vs. other consequences of rhabdomyolysis) is its trigger; and (c) ascertain some of its subcellular determinants. Rats were injected with a borderline toxic dose of glycerol and 24 hours later proximal tubular segments (PTS) were isolated for study. Their vulnerability to diverse forms of injury (FeSO4-induced oxidant stress, hypoxia, Ca2+ ionophore, cytochalasin D, PLA2) was compared to that found in normal PTS. Post-glycerol PTS manifested significant resistance to each insult (decreased lactate dehydrogenase ± N-acetyl-β-D-glucosaminidase release). Protection against FeSO4 was virtually complete and it was associated with a 50% decrease in membrane lipid peroxidation. No decrease in hydroxyl radical generation was noted during the FeSO4 challenge (salicylate trap assessment), suggesting a primary increase in membrane resistance to attack. That PLA2 addition caused less deacylation, plasma membrane enzyme (alanine aminopeptidase) release, and LDH leakage from post-glycerol versus normal tubules supported this hypothesis. To test whether cytoresistance was specifically triggered by heme proteins (vs. being a non-specific filtered protein effect, or a result of endotoxin cascade activation), rats were injected with purified myoglobin, non-heme containing filterable proteins, or endotoxin. Only myoglobin induced cytoresistance. In vivo heme oxygenase inhibition (tin-protoporphyrin) did not block the emergence of cytoresistance and it was expressed despite Na,K-ATPase inhibition (ouabain) or cytoskeletal disruption (cytochalasin D). In vivo heat shock failed to protect. In conclusion, (1) rhabdomyolysis induces broad based proximal tubular cytoresistance; (2) heme proteinuria is its trigger; and (3) it is most easily explained by a primary increase in plasma membrane resistance to attack.