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+.
Ludewig et al. Genome Biology 2001 2:reviews1010.1 doi:10.1186/gb-2001-2-3-reviews1010