The use of membrane vesicles to study the NaCl/KCl cotransporter involved in active transepithelial chloride transport.

Properties of the NaCl/KCl cotransport system were investigated in isolated membranes by flux measurements and binding studies. Chloride competes with "furosemide-like loop diuretics" for its two binding sites at the cotransporter as evidenced by the decrease in piretanide sensitivity of sodium flux and inhibition of high affinity N-methylfurosemide binding by chloride in rectal gland plasma membranes. In the rectal gland lithium inhibits sodium flux but is not translocated whereas in the renal thick ascending limb (TALH) it is also transported. Ammonium is a substrate for the sodium and potassium site in the rectal gland but only for the potassium site in the TALH. The latter finding raises the possibility that part of the ammonium reabsorption in the TALH is mediated by the cotransport system as NaCl/NH4Cl cotransport.