Figure 3. Model proposed for filling of the CVC in protists.

Ammonia (NH₃) is transported to the CVC where it combines with H⁺ pumped in by the action of the vacuolar H⁺-ATPase (which is inhibited by bafilomycin A₁) to generate ammonium (NH₄⁺), which is charged and retained. An anion exchanger (AE, which is inhibited by H₂DIDS) transports HCO₃⁻ in exchange of Cl⁻, which enters the vacuole through a chloride channel. NH₄⁺ and HCO₃⁻ are osmolytes that attract water, which enters through an aquaporin (AQP) and swells the CV. The CV contacts the plasma membrane (kiss and run) through a porosome, releasing water to the medium. A carbonic anhydrase (which is inhibited by acetazolamide, ACZ) generates CO₂ from HCO₃⁻ and H⁺ (pumped in by the V-H⁺-ATPase). CO₂ is released into the cytosol through the protein RH50, while ammonium is released into the cytosol through an ammonium transporter (AmtB).