Abstract
Ochratoxin A (OTA) is a widespread nephrotoxin excreted to a substantial degree via the kidney. Previously we showed that [3H]OTA can be reabsorbed along the rat nephronin vivo (Zingerle et al., 1997). In this study we investigated in detail the contribution of different nephron segments to [3H]OTA reabsorption and determined the possible mechanisms involved by microinfusion and microperfusion experiments. At pH 6 (∼94% of OTA neutral), OTA is reabsorbed in all nephron segments investigated. The estimated fractional reabsorptions (FR) at a tubular load of 20 fmol/min are: proximal convoluted tubule (PCT), 14.8%; proximal straight tubule (PST), 27.4%; ascending limb of Henle′s loop (ALH), 13.6%; distal tubule (DT), 11.6%; collecting duct (CD), 24.6%; terminal CD, 22.0%. At pH 8 (∼10% of OTA neutral) FR are as follows: PCT, 0%; PST, 25.9%; ALH, 14.0%; DT, 3.2%; CD, 8.2%. Thus, OTA reabsorption in PST and ALH is pH-independent. Reabsorption in PST but not in DT or CD was inhibited by sulfobromophthalein, a substrate of the apical organic anion carrier. l-Phenylalanine did not reduce OTA reabsorption. After intravenous injection of unlabeled OTA, resulting in a plasma concentration of ∼10−5 mol/l, the FR of [3H]OTA during early proximal microinfusion was reduced slightly. From our results we conclude: 1) OTA can be reabsorbed in all nephron segments investigated. 2) Under physiological conditions the predominant sites of reabsorption are PST, ALH and terminal CD. 3) Reabsorption in PST and ALH is not pH-dependent. 4) pH-independent reabsorption in PST is mediated by the apical organic anion transporter (OAT-K1), whereas pH-dependent reabsorption in PCT is mediated by H+-dipeptide cotransporter(s). 5) Reabsorption also takes place during natural exposure, i.e., when OTA is present in plasma and renal tissue. 6) The high FR in ALH and CD explains, at least in part, the preferential impairment of postproximal functions and the accumulation in renal inner medulla and papilla.
Footnotes
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Send reprint requests to: PD Dr. Michael Gekle, Physiologisches Institut, Röntgenring 9, D-97070 Würzburg, Germany.
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↵1 This study was supported by the Deutsche Forschungsgemeinschaft (DFG Si 170/7–2 and Ge 905/3–3) and by U.S. National Institutes of Health Research Grant DK 16294.
- Abbreviations:
- OTA
- ochratoxin A
- FE
- fractional excretion
- FR
- fractional reabsorption
- TES
- N-tris(hydroxymethyl)methyl-2-aminomethane sulfonic acid
- MES
- 2-(N-morpholino)ethanesulfonic acid
- PCT
- proximal convoluted tubule
- PST
- proximal straight tubule
- ALH
- ascending limb of Henle′s loop
- DT
- distal tubule
- CD
- collecting duct
- EP
- early proximal
- LP
- late proximal
- ED
- early distal
- LD
- late distal
- LH
- loop of Henle
- BSP
- sulfobromophthalein
- OAT-K1
- organic anion transporter K1
- Received October 17, 1997.
- Accepted March 9, 1998.
- The American Society for Pharmacology and Experimental Therapeutics
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