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

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.