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 micrograms ml-1. 2. At a concentration of 5 micrograms ml-1, a ratio of renal clearance over clearance by glomerular filtration (ClR/GF) of 0.63 +/- 0.06 could be determined. This ratio increased until 1.02 +/- 0.06 at 20 micrograms 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, KA, of 7.3 +/- 1.3 micrograms ml-1, and a maximum rate of absorption, VA,Max, of 1.4 +/- 0.1 micrograms 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.