Fig. 9.

Model of collecting duct ammonia secretion. Interstitial NH₄⁺ is in equilibrium with NH₃ and H⁺. NH₃ is transported across the basolateral membrane through both Rhbg and Rhcg. In the inner MCD (IMCD), basolateral Na⁺-K⁺-ATPase contributes to NH₄⁺ transport; NH₄⁺ then dissociates to NH₃ and H⁺ (black dotted line). Intracellular NH₃ is secreted across the apical membrane by apical Rhcg. H⁺-ATPase and H⁺-K⁺-ATPase secrete H⁺; H⁺ combines with luminal NH₃ to form NH₄⁺, which is “trapped” in the lumen. In addition, there are also components of diffusive NH₃ movement across both the basolateral and apical plasma membranes (gray dotted lines). 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.