FIGURE 1.

Overview of the main apical transport pathways contributing to Na⁺ reabsorption and K⁺ secretion in the ASDN. In the initial two-thirds of the DCT (DCT1), the apical NCC is the major Na⁺ transporter, which mediates electroneutral Na⁺ reabsorption. In the final one-third of the DCT (DCT2), NCC is co-expressed with the ENaC and ROMK. K⁺ secretion progressively increases in the principal cells of DCT2, CNT and CCD that express ENaC and ROMK. Na⁺ reabsorption by ENaC is electrogenic and is regulated by aldosterone. Na⁺ reabsorption through ENaC generates a lumen-negative transepithelial voltage that drives K⁺ secretion via ROMK. BK channels mediate flow-dependent K⁺ secretion in the collecting duct. The three major factors that promote K⁺ secretion are (i) Na⁺ delivery to CNT/CCD, (ii) tubular flow rate and (iii) aldosterone.