Purpose: The mechanism of excretion of the anthelmintic drug ivermectin was investigated in a novel experimental model of functionally intact proximal tubules isolated from a teleost fish (Fundulus heteroclitus).
Methods: Secretion into the lumens of freshly isolated proximal tubules was studied by means of confocal laser scanning microscopy and digital image analysis using ivermectin and fluorescent labelled ivermectin (BODIPY-ivermectin; BI) as substrates.
Results: The tubular cells rapidly accumulated BI from the medium and attained steady state within 25 minutes. Luminal fluorescence in the steady state was 5-7 times higher as compared to intracellular fluorescence. The secretion of BI into the tubular lumens was inhibited in a dose dependent manner by unlabelled ivermectin and inhibitors of the renal excretory membrane pump p-glycoprotein, namely SDZ PSC-833 and verapamil, but not by leukotriene C4, a substrate of the renal export protein mrp2. Accumulation inside the tubular cells was not affected by the added inhibitors. Ivermectin inhibited the renal secretion of the fluorescent cyclosporin derivative NBDL-CS, a substrate of p-glycoprotein, but not the secretion of the mrp2-substrate fluorescein-methotrexate, nor the secretion of fluorescein, a substrate of the classical renal organic anion transporter.
Conclusions: The data are consistent with BI and ivermectin interacting in teleost kidney tubules exclusively with p-glycoprotein, but not with one of the other known excretory transport systems. In addition, the studies demonstrate that freshly isolated functionally intact kidney tubules from killifish are a useful tool to differentiate the substrate specificity of renal transport systems with respect to drug elimination.