Characterization of the thiazide-sensitive Na+-Cl- cotransporter:: a new model for ions and diuretics interaction

The thiazide-sensitive Na+-Cl- cotransporter (TSC) is the major pathway for salt reabsorption in the apical membrane of the mammalian distal convoluted tubule. When expressed in Xenopus laevis oocytes, rat TSC exhibits high affinity for both cotransported ions, with the Michaelis-Menten constant (K-m) for Na+ of 7.6 +/- 1.6 mM and for Cl- of 6.3 +/- 1.1 mM, and Hill coefficients for Na+ and Cl- consistent with electroneutrality. The affinities of both Na+ and Cl- were increased by increasing concentration of the counterion. The IC50 values for thiazides were affected by both extracellular Na+ and Cl-. The higher the Na+ or Cl- concentration, the lower the inhibitory effect of thiazides. Finally, rTSC function is affected by extracellular osmolarity. We propose a transport model featuring a random order of binding in which the binding of each ion facilitates the binding of the counterion. Both ion binding sites alter thiazide-mediated inhibition of transport, indicating that the thiazide-binding site is either shared or modified by both Na+ and Cl-.