Ammonia excretion in the freshwater planarian Schmidtea mediterranea.
In aquatic invertebrates, metabolic nitrogenous waste is excreted predominately as ammonia. Very little is known, however, of the underlying mechanisms of ammonia excretion, particularly in freshwater species. Our results indicate that in the non-parasitic freshwater planarian Schmidtea mediterranea, ammonia excretion depends on acidification of the apical unstirred layer of the body surface and consequent ammonia trapping. Buffering of the environment to a pH of 7 or higher decreased the excretion rate. Inhibitor experiments suggested further that the excretion mechanism involves the participation of the V-type H(+)-ATPase and carbonic anhydrase and possibly also the Na(+)/K(+)-ATPase and Na(+)/H(+) exchangers. Alkalinization (pH 8.5, 2 days) of the environment led to a 1.9-fold increase in body ammonia levels and to a downregulation of V-ATPase (subunit A) and Rh-protein mRNA. Further, a 2 day exposure to non-lethal ammonia concentrations (1 mmol l(-1)) caused a doubling of body ammonia levels and led to an increase in Rh-protein and Na(+)/K(+)-ATPase (α-subunit) mRNA expression levels. In situ hybridization studies indicated a strong mRNA expression of the Rh-protein in the epidermal epithelium. The ammonia excretion mechanism proposed for S. mediterranea reveals striking similarities to the current model suggested to function in the gills of freshwater fish.
Department of Biological Sciences, University of Manitoba, 190 Dysart Road, Winnipeg, MB, R3T 2N2, Canada. weihrauc@cc.umanitoba.ca
Dirk
D
Weihrauch
Ainsely C
AC
Chan
Heiko
H
Meyer
Carmen
C
Döring
Mary
M
Sourial
Michael J
MJ
O'Donnell
D000086:Acetazolamide / Q000494:pharmacology; D000641:Ammonia / Q000378:metabolism* / Q000494:pharmacology; D000818:Animals; D004781:Environmental Exposure; D005247:Feeding Behavior / Q000187:drug effects; D029941:Fish Proteins / Q000378:metabolism; D005618:Fresh Water; D005786:Gene Expression Regulation / Q000187:drug effects; D006863:Hydrogen-Ion Concentration / Q000187:drug effects; D008954:Models, Biological; D010932:Planarians / Q000187:drug effects / Q000201:enzymology / Q000235:genetics / Q000378:metabolism*; D021122:Protein Subunits / Q000378:metabolism; D012333:RNA, Messenger / Q000235:genetics / Q000378:metabolism; D060888:Real-Time Polymerase Chain Reaction; D013997:Time Factors; D025262:Vacuolar Proton-Translocating ATPases / Q000378:metabolism
D029941:Fish Proteins; D021122:Protein Subunits; D012333:RNA, Messenger; D000641:Ammonia; D025262:Vacuolar Proton-Translocating ATPases; D000086:Acetazolamide