- Collecting tubule - reabsorbs Na⁺ in exchange for secreting K⁺ and H⁺ (regulated by aldosterone).
- Aldosterone - acts on mineralocorticoid receptor → mRNA → protein synthesis. In principal cells: ⬆️ apical K⁺ conductance, ⬆️ Na⁺/K⁺ pump, ⬆️ epithelial Na⁺ channel (ENaC) activity → lumen negativity → K⁺ secretion. In α-intercalated cells: lumen negativity → ⬆️ H⁺ ATPase activity → ⬆️ H⁺ secretion → ⬆️ HCO₃⁻/CI⁻ exchanger activity.
- ADH - acts at V₂ receptor → insertion of aquaporin H₂O channels on apical side.
- 3-5% Na⁺ reabsorbed.
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