Electroneutral NaCl transport by distal tubule: evidence for Na+/H+-Cl-/HCO3- exchange.

The mechanisms, of electrolyte transport by isolated and perfused late distal tubules of the salamander, Amphiuma, were investigated by electrophysiological and transport techniques. The tubules absorbed Na+, HCO3-, and Cl- but not K+. The transepithelial voltage (VT) was not different from zero. Amiloride (10(-3) M) in the perfusion fluid reduced sodium absorption by 43% and HCO3- absorption by 49% without changing VT. This and previous data are consistent with the presence of a Na+/H+ antiporter in the apical membrane. Hydrochlorothiazide (HCTZ, 10(-4) M) in the perfusion fluid inhibited Na+ absorption by 48% but had no effect on HCO3- absorption or VT. Thus HCTZ reduced NaCl absorption. Intracellular microelectrode techniques were used to examine the cellular mechanisms of ion transport and sites of action of amiloride and HCTZ. Two cell types were identified by their electrophysiological properties. Neither amiloride nor HCTZ appreciably altered the electrical properties of cell type I, a cell previously identified as being involved in H+ secretion. In contrast, both diuretics hyperpolarized the basolateral membrane voltage (Vbl) of cell type II. Additional studies of cell type II showed that the removal of Cl- from the lumen hyperpolarized Vbl, as did the addition to the lumen of the Cl-/HCO3- exchange inhibitor 4,4'-diisothiocyanostilbene-2-2'-disulphonic acid. Finally, reducing the [HCO3-] of the lumen depolarized Vbl.(ABSTRACT TRUNCATED AT 250 WORDS)