Figure 6. Model of collecting duct ammonia secretion.

Ammonia uptake across the basolateral plasma membrane primarily involves transporter-mediated uptake by either Rhbg or Rhcg, with a component of diffusive NH₃ absorption. Cytosolic NH₃ is transported across the apical plasma membrane by a combination of Rhcg and diffusive transport. Not shown is that in the IMCD, but not the CCD, basolateral Na⁺-K⁺-ATPase also contributes to basolateral plasma membrane NH₄⁺ uptake. Cytosolic H⁺ is generated by a carbonic anhydrase-II mediated mechanism, and is secreted across the apical plasma membrane via H⁺-ATPase and H⁺-K⁺-ATPase. Luminal H⁺ titrates luminal NH₃, forming NH₄⁺ and maintaining a low luminal NH₃ concentration necessary for NH₃ secretion. Figure from [35] with permission.