title: Cutaneous nitrogen excretion in the African clawed frog Xenopus laevis: effects of high environmental ammonia (HEA).
abstract: 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.
authors.affiliation: Biology Faculty, University of British Columbia, Kelowna, BC V1V 1V7, Canada.
authors.forename: Melissa J
authors.initials: MJ
authors.lastname: Cruz
authors.forename: Mary M
authors.initials: MM
authors.lastname: Sourial
authors.forename: Jason R
authors.initials: JR
authors.lastname: Treberg
authors.forename: Sandra
authors.initials: S
authors.lastname: Fehsenfeld
authors.forename: Aida
authors.initials: A
authors.lastname: Adlimoghaddam
authors.forename: Dirk
authors.initials: D
authors.lastname: Weihrauch
mesh_terms: D000641:Ammonia / Q000032:analysis*; D000704:Analysis of Variance; D000818:Animals; D001692:Biological Transport / Q000187:drug effects / Q000502:physiology; D017931:DNA Primers / Q000235:genetics; D004781:Environmental Exposure; D008297:Male; D008954:Models, Biological; D009584:Nitrogen / Q000378:metabolism*; D060888:Real-Time Polymerase Chain Reaction; D012867:Skin / Q000378:metabolism*; D000254:Sodium-Potassium-Exchanging ATPase / Q000378:metabolism; D013806:Theophylline / Q000494:pharmacology; D014508:Urea / Q000378:metabolism; D014982:Xenopus laevis / Q000502:physiology*
chemical_list: D017931:DNA Primers; D000641:Ammonia; D014508:Urea; D013806:Theophylline; D000254:Sodium-Potassium-Exchanging ATPase; D009584:Nitrogen