Tissue concentration and urinary excretion pattern of sulfofluorescein by the rat kidney

In order to find a fluorescein analog that is excreted in a way similar to p-aminohippurate (PAH) and is suitable to register excretion into the urine and also to monitor continuously the concentration within cortical tissue, the interference of fluorescein, sulfofluorescein (SF), and fluorescein-5(6)-sulfonate with the contraluminal transport systems of PAH, succinate, and sulfate and with the luminal transport system of sulfate and lactate was evaluated. All three substances exerted a strong inhibitory potency against contraluminal PAH uptake (apparent Ki, 0.06 to 0.1 mmol/L) and also showed a moderate to small inhibitory potency against contraluminal sulfate transport (apparent Ki, 0.7 to 5.3 mmol/L). None of the three substrates interacted with the contraluminal dicarboxylate transport. Luminally, fluorescein and SF interacted with the lactate transporter (apparent Ki, approximately 3.0 mmol/L), whereas SF and fluorescein-5(6)-sulfonate had a very weak inhibitory potency against luminal sulfate transport (apparent Ki, 30 to 40 mmol/L). Because of its relatively low interference with the contraluminal sulfate transport, we preferred SF in the following study (protein binding, 88%) over fluorescein-5(6)-sulfonate (protein binding, 43.3%) and fluorescein (protein binding, 77.3%). A bolus injection of SF was given together with [14C]inulin into the jugular vein to rats in mannitol diuresis. Excretion of both substances in the urine was measured in 5-min samples. Fluorescence of the kidney surface was monitored at the exposed kidney with a photocell (excitation light, 470 nm; emission light, 530 nm). The effect of interfering substances was evaluated by their application 1 min before, simultaneously, or 1 min after SF bolus injection. PAH < probenecid < apalcillin inhibited SF excretion in the urine and, to a similar degree, SF fluorescence in cortical tissue. Inhibition was strongest when the substances were given simultaneously with SF. With the injection of alpha-ketoglutarate, glutarate, and succinate, an increase of SF excretion in the urine and, partially also, of SF content in tissue was seen. Tetrafluorosuccinate and mercaptosuccinate inhibited urinary SF excretion and tissue fluorescence strongly. Therefore, SF secretion is completely inhibited whereas filtration remained unaffected. The injection of acetate, lactate, and pyruvate as well as of 2-chloropropionate and thiosulfate did not change urinary excretion and tissue fluorescence of SF signifi