The role of altered mitochondrial function in citrinin-induced toxicity to rat renal proximal tubule suspensions

Citrinin (CTN), a mycotoxin produced by several species of Penicillium and Aspergillus, causes renal proximal tubule (RPT) cell injury and death by an unknown mechanism of action. Using suspensions of rat RPT, the cellular events preceding CTN-induced cytotoxicity were investigated. Tubule viability decreased in a concentration- and time-dependent manner after CTN exposure, with cell death beginning 1, 2, and 4 hr after exposure to 500, 125-250, and 63 μm CTN, respectively. Basal oxygen consumption (QO₂) of RPT increased from 41 to 53 nmol O₂ · mg protein^-1 · min^-1 30 min after exposure to 250 μm CTN and returned to control values 1 hr after exposure. A similar concentration- and time-dependent transitory rise in basal QO₂ occurred at all concentrations of CTN tested (63-500 μm). Nystatin-stimulated QO₂, an indirect measure of mitochondrial state 3 respiration in RPT, decreased 11% at 0.5 and 1 hr after exposure to 500 and 250 μm CTN, respectively, but was not affected after exposure to 63 and 125 μm CTN. Adenosine triphosphate content declined 22% to 48% in RPT at 0.5 and 1.5 hr after exposure to 500 and 125-250 μm CTN, respectively. Although lipid peroxidation occurred concurrently with RPT cell death, iron-mediated oxidative stress was not a causative factor in the development of toxicity since pretreatment with 1 mm deferoxamine prevented ironmediated lipid peroxidation but did not protect RPT from CTN-induced cell death. Further studies using RPT and isolated renal cortical mitochondria (RCM) showed that CTN had multiple effects on mitochondrial function. Direct probing of mitochondrial function within RPT showed that a 1-hr exposure to 250 μm CTN increased spontaneous respiration 55% in RPT respiring on the site I respiratory substrates glutamate/malate while state 3 respiration decreased 34%. CTN also decreased succinate supported respiration but had no effect on cytochrome c-cytochrome oxidase. With isolated RCM, a 3-min exposure to 125 and 250 μm CTN increased state 4 respiration in the absence of a phosphate acceptor 27 and 67%, respectively, while 250 μm CTN decreased state 3 respiration 23%. Respiration in the presence of a known uncoupler was reduced after CTN exposure (63-250 μm) in a concentration-dependent manner. These results indicate that CTN has multiple effects on mitochondrial function in RPT and isolated RCM which may contribute to the development of cell death in rat RPT.