Figure 1.

Ion and water transporters in CD principal cells. Sodium reabsorption occurs sequentially through apical ENaC and basolateral Na⁺,K⁺-ATPase. This vectorial Na⁺ transport generates a lumen-negative transepithelial potential that drives potassium secretion via luminal K⁺-channels, i.e., ROMK and Big K⁺-channels (BK). The Na⁺,K⁺-ATPase generates the driving forces for both Na⁺ entry and K⁺ exit at the apical side of the cell. Basolateral K⁺ influx through Na⁺,K⁺-ATPase is also equilibrated by K⁺ efflux through basolateral K⁺-channels (Kir) to generate the negative resting transmembrane potential. Water permeability is mediated by sequential flow through apical AQP2 water channels and basolateral aquaporin-3 and -4 water channels.