Fig. 4.

Ammonia transport in the proximal tubule. Ammonia is produced in the proximal tubule primarily from metabolism of glutamine and occurs primarily in the mitochondria. The enzymatic details of ammoniagenesis are not shown. Three transport mechanisms appear to mediate preferential apical ammonia secretion. These include Na⁺/NH₄⁺ exchange via NHE-3, parallel NH₃ secretion and NHE-3-mediated Na⁺/H⁺ exchange, and a Ba²⁺-sensitive NH₄⁺ conductance likely mediated by apical K⁺ channels. HCO₃⁻ is produced in equimolar amounts as NH₄⁺ in the process of ammoniagenesis and is primarily transported across the basolateral plasma membrane by NBCe1. Minor components of basolateral NH₄⁺ uptake via Na⁺-K⁺-ATPase and by basolateral K⁺ channels are not shown.