Thiazide-sensitive Na-Cl cotransport mediates NaCl absorption in amphibian distal tubule

To find out the mechanism(s) underlying NaCl absorption in the distal tubule of Necturus, we devised a variant of the split-drop technique. Following injection an oil column, subsequently split by a NaCl solution isotonic to plasma, a double-barrelled microelectrode (conventional/selective to Na⁺ or to Cl^– ions) recorded Na⁺ (_Na) or Cl^– (_Cl) activity and transepithelial potential (V_te). Paired control/low-Na⁺ solutions yielded reabsorptive half-times (t_1/2) of 0.68±0.11 min and 7.6±1.8 min respectively; corresponding V_te values were –22.2±4.0 mV and –7.6±1.9 mV. t_1/2 values of control versus low-Cl^– solutions were 0.77±0.32 min and 6.5±1.7 min respectively, whereas respective V_te values were not different from one another: –23.8±4.3 mV versus –18.8±5.5 mV. Nominally K⁺-free solutions or bumetanide, 10 mol/l, did not alter t_1/2 or V_te, with regard to the paired control. Amiloride, 5 mol/l or 2 mmol/l, failed to decrease t_1/2 or to lower V_te; apparently, the role of a Na⁺/H⁺ antiport does not contribute significantly to NaCl absorption. Furosemide, 0.1 mmol/l, reduced t_1/2 by 54% with regard to the control state. Determination of t_1/2 as a function of increasing hydrochlorothiazide concentrations revealed apical high- and low-affinity sites, estimated at 0.56 mol/l and 0.115 mmol/l respectively. Taken together these observations indicate that NaCl absorption is predominantly carried out by an electroneutral Na⁺-Cl^– cotransport.