Fig. 5.

Ammonia reabsorption in the TAL. The primary mechanism of ammonia reabsorption in the TAL is via substitution of NH₄⁺ for K⁺ and transport by NKCC2. Electroneutral K⁺/NH₄⁺ exchange and conductive K⁺ transport are also present, but are quantitatively less significant components of apical K⁺ transport. Diffusive NH₃ transport across the apical plasma membrane is present, but is not quantitatively significant. Cytosolic NH₄⁺ can exit via basolateral NHE-4. A second mechanism of basolateral NH₄⁺ exit may involve dissociation to NH₃ and H⁺, with NH₃ exit via an uncharacterized, presumably diffusive, mechanism and buffering of intracellular H⁺ released via sodium-bicarbonate cotransporter NBCn1-mediated HCO₃⁻ entry.