Sodium entry mechanisms in distal convoluted tubule cells

Sodium transport across apical membranes of distal convoluted tubules is thought to be mediated by Na-Cl cotransport and conductive Na entry. Immortalized mouse distal convoluted tubule cells were used to characterize Na entry pathways. Chlorothiazide, an inhibitor of Na-Cl cotransport, and amiloride, which blocks epithelial Na channels, reduced ouabain-suppressible oxygen consumption by 40 and 35%, respectively. In simple buffer solutions, free of bicarbonate, phosphate, or formate, chlorothiazide inhibited Na uptake by 44% and Cl uptake by 48%. Michaelis constants of 21 mM for Na and 14 mM for chloride were calculated. Amiloride inhibited Na uptake by 49% and had no effect on Cl uptake. The calculated Hill coefficient of 1.07 and the equivalence of chlorothiazide-sensitive Na and Cl uptake are consistent with the presence of Na-Cl cotransport. Na-Cl cotransport and amiloride-sensitive Na influx account for 85% of Na entry in distal convoluted tubule cells in the absence of phosphate and formate. The selective Na/H exchange inhibitor ethylisopropyl amiloride had no effect on Na uptake; however, it abolished formate-stimulated Na uptake. The anion exchange blocker 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS) inhibited Na uptake. These findings are consistent with parallel Na/H and Cl/formate exchange. Na uptake was inhibited 8% by the selective Na/Ca exchange inhibitor, dimethylbenzamil. An additional 7% of Na entry was phosphate dependent and was abolished by phosphonoformic acid, a competitive inhibitor of Na-Pi cotransport. In summary, the majority of Na entry into distal convoluted tubule cells occurs through Na-Cl cotransport and an amiloride-sensitive pathway (75% in presence of phosphate and formate). An additional 13% may enter by Na/H exchange, with the remainder mediated by Na/Ca exchange and Na-Pi cotransport.