Mercury blockade of thiazide-sensitive NaCl cotransport in flounder urinary bladder

The effects of HgCl₂ on ion transport were investigated using isolated sheets of flounder urinary bladder, a model epithelium that is capable of electrically silent NaCl absorption and electrogenic K secretion. Exposure of the mucosal surface of the bladder to submicromolar doses of HgCl₂ reduced K secretion, but the effect was not due to blockade of apical K channels. Rather, the effects of HgCl₂ were virtually identical to those seen with experimental maneuvers that blocked the thiazide-sensitive NaCl cotransporter in the apical membrane, e.g., hydrochlorothiazide, Cl-free solutions, and Na-free solutions. Mucosal HgCl₂ also blocked ²²Na absorption, suggesting that the effect of the metal was mediated by blockade of NaCl entry. The effects of HgCl₂ had a rapid onset and were readily reversed by washing, suggesting a noncovalent binding reaction. The abundance of polyanionic Hg complexes in salt solutions prompts the speculation that one of these may bind to the Cl-binding site on the cotransporter, thereby blocking it. The results provide the first evidence that the thiazide-sensitive NaCl cotransporter is a specific site of action for inorganic mercury.