Figure 9.

Ammonia transport in the collecting duct. Interstitial NH₄⁺ is in equilibrium with NH₃ and H⁺. NH₃ is transported across the basolateral membrane, predominantly by Rhcg, but also possibly partly by Rhbg. In the inner medullary collecting duct, basolateral Na⁺-K⁺-ATPase transports NH₄⁺. Intracellular NH₃ is secreted across the apical membrane by apical Rhcg. H⁺-ATPase and H⁺-K⁺-ATPase secrete H⁺, which combines with luminal NH₃ to form NH₄⁺ which is “trapped” in the lumen. Intracellular H⁺ is generated by CA II-accelerated CO₂ hydration that forms carbonic acid, which dissociates to H⁺ and HCO₃⁻. Basolateral Cl⁻/HCO₃⁻ exchange transports HCO₃⁻ across the basolateral membrane; HCO₃⁻ combines with H⁺ released from NH₄⁺ to form carbonic acid, which dissociates to CO₂ and water. This CO₂ can recycle into the cell, supplying the CO₂ used for cytosolic H⁺ production. The net result is NH₄⁺ transport from the peritubular space into the luminal fluid. Figure modified from reference (180) with permission.