Fig. 7.

Directions of NH3 and NH₄⁺ transport across the apical and basolateral plasma membrane of collecting duct cells. Model shows electrochemical gradients for NH₃ and NH₄⁺ movement across apical and basolateral plasma membranes of collecting duct cells. Because NH₃ is uncharged, membrane voltage does not directly affect NH₃ transport. As a result, NH₃ movement from the interstitium into the cell cytoplasm and across the apical plasma membrane is driven solely by the chemical concentration gradients. For NH₄⁺, electrochemical gradients, driven largely by intracellular electronegativity, favor NH₄⁺ uptake across the basolateral plasma membrane. Across the apical plasma membrane, intracellular electronegativity would facilitate lumen-to-cytoplasm NH₄⁺ movement. Consequently, an electrogenic NH₄⁺ transport mechanism is highly unlikely to be able to contribute to collecting duct ammonia secretion. Arrows show likely movements of NH₃ and NH₄⁺ transport across apical and basolateral plasma membranes given typical total ammonia concentrations and pH in interstitium, cytoplasm, and lumen and cell membrane voltage. Arrows in green shown transport consistent with known ammonia movement, and arrow in red, showing luminal NH₄⁺ reabsorption, shows transport unlikely to contribute to ammonia secretion.