Nephrotoxicity assessment by measuring cellular ATP content. II. Intranephron site of ochratoxin A nephrotoxicity

To clarify the nephrotoxic site and potency of ochratoxin A (OCTA), we measured cellular ATP contents in nine nephron segments incubated with or without OCTA in vitro. Cellular ATP contents of nephron segments isolated under stereomicroscopic observation after treatment of renal slices with 0.1% collagenase were measured by the microchemiluminescence method. OCTA decreased cellular ATP content in a dose-dependent manner. A concentration-response study of OCTA showed that the minimum concentration of OCTA needed to cause a significant decrease in ATP was 10(-8) M in the middle portion of the proximal tubule (S2; p less than 0.05) and 5 x 10(-4) M in the medullary collecting tubule (MCT; p less than 0.01). Among nine nephron segments, OCTA at 5 x 10(-5) M significantly decreased cellular ATP content in only S2 and the terminal portion of the proximal tubule (S3; p less than 0.01). ATP synthesis in mitochondria isolated from the renal cortex was significantly inhibited by 10(-6) M OCTA (p less than 0.05). Probenecid at 4 x 10(-4) M protected against the OCTA-induced cellular ATP decrease. These results suggest that OCTA might enter the plasma membrane in S2 and S3 through the organic anion transport pathway and inhibit mitochondrial oxidative phosphorylation. This newly established method would be applicable to evaluation of the intrarenal toxic site and potency of various chemical compounds.