Figure 1

Models for NH₃ and NH₄⁺ transport. (a) Channel- or carrier-mediated uniport for NH₄⁺. Both concentration gradient and transmembrane voltage influence the direction of transport. A firmly established example of such NH₄⁺ transport is through potassium channels. (b) NH₃/H⁺ co-transporter. Driven by the pH gradient and transmembrane voltage, ammonia may be taken up against a concentration gradient for NH₃. The electrochemical situation is similar as for NH₄⁺ uniport in (a). (c) Carrier-mediated antiport for NH₄⁺ with unknown secondary substrate(s), for example, cation C⁺. If both substrates are univalent only the concentration gradients determine the direction of transport. In the kidney, C⁺ could be sodium. (d) Channel- or carrier-mediated uniport for NH₃. Independent of the membrane potential, ammonia will equilibrate along its concentration gradient (depending on the external and internal pH). A net inflow of ammonium into the cytoplasm will depend on ammonium metabolism, which acts as a sink for NH₃/NH₄⁺.