Cutaneous nitrogen excretion in the African clawed frog Xenopus laevis: Effects of high environmental ammonia (HEA)

• The skin of Xenopus laevis generates a net-efflux of ammonia.
• Sub-lethal environmental NH4Cl concentrations reduce cutaneous ammonia transport.
• Prolonged ammonia exposure reduces mRNA expression of Rhbg, Rhcg, Na/K-ATPase and V-ATPase.
• X. laevis Na/K-ATPase does accept NH4+ as a substrate.

Ammonia is a highly toxic molecule and often introduced in considerable amounts into aquatic environments due to anthropogenic activities. Many aquatic and semi-aquatic amphibians utilize, in addition to their kidneys, the skin for osmoregulation and nitrogen excretion. In the present study the effects of prolonged (7–21 days) exposure to high environmental ammonia (HEA, 1 mmol l−1 NH4Cl) on cutaneous nitrogen excretion and gene expression of key-transporters involved in nitrogen excretion and acid-base regulation were investigated in the fully aquatic African clawed frog, Xenopus laevis. The study revealed that X. laevis excretes predominately ammonia of which approximately 50% is excreted via the skin. Both the ventral and dorsal skin were capable to generate a net ammonia efflux, which was significantly activated by 10 mmol l−1 of the phosphodiesterase blocker theophylline. The obtained data further suggest that the ammonia efflux was promoted by an acidification of the unstirred boundary layer, likely generated by an apical localized V-ATPase, with NH3 being transported via cutaneous expressed ammonia transporters, Rhbg and Rhcg. Prolonged HEA exposure did significantly reduce the net-flux rates over the ventral skin with Vmax changing from 256 nmol cm−2 h−1 in control frogs to 196 nmol cm−2 h−1 in HEA exposed animals. Further, prolonged HEA exposure caused a decrease in mRNA expression levels of the ammonia transporter Rhbg, Na+/K+-ATPase (α-subunit) and V-ATPase (subunit H) in the ventral and dorsal skin and the kidney. In contrast, Rhcg expression levels were unaffected by HEA in skin tissues.