Sodium uptake in different life stages of crustaceans: the water flea Daphnia magna Strauss.
The concentration-dependent kinetics and main mechanisms of whole-body Na+ uptake were assessed in neonate and adult water flea Daphnia magna Strauss acclimated to moderately hard water (0.6 mmol l(-1) NaCl, 1.0 mmol l(-1) CaCO3 and 0.15 mmol l(-1) MgSO4.7H2O; pH 8.2). Whole-body Na+ uptake is independent of the presence of Cl(-) in the external medium and kinetic parameters are dependent on the life stage. Adults have a lower maximum capacity of Na+ transport on a mass-specific basis but a higher affinity for Na+ when compared to neonates. Based on pharmacological analyses, mechanisms involved in whole-body Na+ uptake differ according to the life stage considered. In neonates, a proton pump-coupled Na+ channel appears to play an important role in the whole-body Na+ uptake at the apical membrane. However, they do not appear to contribute to whole-body Na+ uptake in adults, where only the Na+ channel seems to be present, associated with the Na+/H+ exchanger. In both cases, carbonic anhydrase contributes by providing H+ for the transporters. At the basolateral membrane of the salt-transporting epithelia of neonates, Na+ is pumped from the cells to the extracellular fluid by a Na+, K+-ATPase and a Na+/Cl(-) exchanger whereas K+ and Cl(-) move through specific channels. In adults, a Na+/K+/2Cl(-) cotransporter replaces the Na+/Cl(-) exchanger. Differential sensitivity of neonates and adults to iono- and osmoregulatory toxicants, such as metals, are discussed with respect to differences in whole-body Na+ uptake kinetics, as well as in the mechanisms of Na+ transport involved in the whole-body Na+ uptake in the two life stages.
Bianchini
Adalto
A
Fundação Universidade Federal do Rio Grande, Departamento de Ciências Fisiológicas, Campus Carreiros, Av. Itália s/n, 96.201-900 Rio Grande, RS, Brazil. adalto@octopus.furg.br
Wood
Chris M
CM
D000818:Animals; D001692:Biological Transport; D003621:Daphnia / Q000187:drug effects / Q000254:growth & development* / Q000378:metabolism*; D008018:Life Cycle Stages / Q000502:physiology*; D012964:Sodium / Q000378:metabolism*; D026941:Sodium Channel Blockers / Q000494:pharmacology
D026941:Sodium Channel Blockers; D012964:Sodium