Fig. 12.

Detailed model of collecting duct ammonia secretion. We propose the following model of collecting duct ammonia secretion. Interstitial NH₄⁺ is in equilibrium with NH₃ and H⁺. NH₃ is transported across the basolateral membrane, predominantly by Rhcg, but also by lipid diffusion (dotted line) and possibly by Rhbg. In the IMCD, basolateral Na⁺-K⁺-ATPase transports NH₄⁺ (dashed line). Intracellular NH₃ is secreted across the apical membrane by apical Rhcg. There is also likely to be a component of diffusive apical NH₃ transport (dotted line). H⁺-ATPase and H⁺-K⁺-ATPase secrete H⁺, which combines with luminal NH₃ to form NH₄⁺, which is “trapped” in the lumen. Apical H⁺-K⁺-ATPase may also contribute to NH₄⁺ secretion (not shown) (21). Intracellular H⁺ is generated by carbonic anhydrase II-accelerated CO₂ hydration, forming carbonic acid, which dissociates to H⁺ and HCO₃⁻. Basolateral Cl⁻/HCO₃⁻ exchange transports the HCO₃⁻ across the basolateral membrane, where it combines with H⁺ released from NH₄⁺, forming 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 interstitium into the luminal fluid.