Glomerular filtration and saturable absorption of iohexol in the rat isolated perfused kidney

1 The renal handling of iohexol was examined in the rat isolated perfused kidney (IPK) over a perfusate concentration range of 5–20 μg ml −1 . 2 At a concentration of 5 μg ml −1 , a ratio of renal clearance over clearance by glomerular filtration (Cl R /GF) of 0.63 ± 0.06 could be determined. This ratio increased until 1.02 ± 0.06 at 20 μg ml −1 , indicating that a saturable mechanism is involved in the luminal disappearance of the drug. 3 Pretreatment of the kidneys with polylysine, probenecid or diatrizoate resulted in a significantly enhanced clearance of iohexol, probably due to inhibition of membrane binding. Renal clearance data were fitted to a kinetic model including filtration into the primary urine followed by saturable absorption at the luminal membrane. An absorption constant, K A , of 7.3 ± 1.3 μg ml −1 , and a maximum rate of absorption, V A, Max , of 1.4 ± 0.1 μg min −1 were determined. 4 Iohexol accumulated in kidney tissue, reaching a concentration of 2 to 7.5 times the perfusate concentration. In freshly isolated proximal tubular cells and kidney cortex mitochondria, iohexol reduced the uncoupled respiratory rate at a concentration comparable to the highest tissue concentration found in the IPK. 5 In conclusion, iohexol is not only filtered by the kidney but also reabsorbed via a saturable mechanism, which results in tubular accumulation. Intracellularly sequestered iohexol may affect mitochondrial oxidative metabolism. Our results indicate that iohexol is not a true filtration marker.