Figure 7.

Current model of putative transcellular ammonia (NH₃/${\text{NH}}_4^{+}$) transport mechanisms in the anal papillae epithelium of A. aegypti larvae in freshwater conditions (adapted from Chasiotis et al., 2016 copyright The Company of Biologists). Basolateral NKA provides a cytosol negative voltage potential that could serve to drive ${\text{NH}}_4^{+}$ from the hemolymph to the cytosol through AeAmt1 (Amt1). Partial pressure gradient (ΔP_NH3/CO2) driven entry of CO₂ and NH₃ into the cytosol occurs through one of the two Rh proteins (AeRh50-1 or AeRh50-2). On the apical side, AeAmt2 facilitates NH₃/${\text{NH}}_4^{+}$ exit from the cytosol to the aqueous habitat either through ${\text{NH}}_4^{+}$ transport, NH₃ transport following ${\text{NH}}_4^{+}$ recruitment and deprotonation, or NH₃/H⁺ co-transport. ${\text{NH}}_4^{+}$ in the cytosol may exit from the apical side to the aqueous habitat through the sodium-hydrogen exchanger 3 (NHE3) in exchange for Na⁺. An Rh protein (Rh) on the apical side may function to excrete NH₃ through an ammonia-trapping mechanism exploiting an acidified boundary layer generated by apical VA and partly by NHE3. Cytoplasmic carbonic anhydrase (CA) contributes to the cytosol negative voltage potential and the maintenance of the acidified boundary layer by supplying H⁺ for VA.