Active xenobiotic excretion by Elasmo-branch rectal salt gland (RG) tubules

Marine elasmobranch RG is a specialized, salt excretory organ composed of numerous blind-ended, branched tubules emptying into a central duct. To date, NaCl excretion has been its only described function. Here we use isolated RG tubules from dogfish shark (Squalus acanthias), fluorescent xenobiotics and confocal microscopy to describe a second function, xenobiotic excretion. Isolated RG tubules rapidly transported a fluorescent organic anion, sulforhodamine 101 (S-101), from bath to lumen. Lumenal (not cellular) accumulation was concentrative, saturable and inhibited by cyclosporine A (CSA), chlorodinitrobenzene (CDNB), leukotriene C4 and KCN. Inhibitors of renal organic anion transport (probenecid, p-aminohippurate), organic cation transport (tetraethylammonium) and p-glycoprotein (verapamil) were without effect. RG tubules did not secrete fluorescein, daunomycin, rhodamine 123 nor a fluorescent CSA derivative. Perfused RG secreted S-101 into duct fluid and secretion was blocked by CDNB. Finally, frozen RG sections stained with antibody to a multispecific organic anion transporter (cMoat or Mrp2) showed heavy and specific staining on the luminal membrane of the epithelial cells. We conclude that RG is capable of active and specific excretion of xenobiotics and that such transport is mediated by Mrp2, an ATP-driven xenobiotic transporter, but not by p-glycoprotein.