Crystal and microparticle effects on MDCK cell superoxide production: oxalate-specific mitochondrial membrane potential changes

We have previously shown that crystals of calcium oxalate (COM) elicit a superoxide (O2−) response from mitochondria. We have now investigated: (i) if other microparticles can elicit the same response, (ii) if processing of crystals is involved, and (iii) at what level of mitochondrial function oxalate acts. O2− was measured in digitonin-permeabilized MDCK cells by lucigenin (10 μM) chemiluminescence. [14C]-COM dissociation was examined with or without EDTA and employing alternative chelators. Whereas mitochondrial O2− in COM-treated cells was three- to fourfold enhanced compared to controls, other particulates (uric acid, zymosan, and latex beads) either did not increase O2− or were much less effective (hydroxyapatite +50%, p < 0.01), with all at 28 μg/cm2. Free oxalate (750 μM), at the level released from COM with EDTA (1 mM), increased O2− (+50%, p < 0.01). Omitting EDTA abrogated this signal, which was restored completely by EGTA and partially by ascorbate, but not by desferrioxamine or citrate. Omission of phosphate abrogated O2−, implicating phosphate-dependent mitochondrial dicarboxylate transport. COM caused a time-related increase in the mitochondrial membrane potential (Δψm) measured using TMRM fluorescence and confocal microscopy. Application of COM to Fura 2-loaded cells induced rapid, large-amplitude cytosolic Ca2+ transients, which were inhibited by thapsigargin, indicating that COM induces release of Ca2+ from internal stores. Thus, COM-induced mitochondrial O2− requires the release of free oxalate and contributes to a synergistic response. Intracellular dissociation of COM and the mitochondrial dicarboxylate transporter are important in O2− production, which is probably regulated by Δψm.