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First published online February 1, 2008
Journal of Experimental Biology 211, 539-547 (2008)
Published by The Company of Biologists 2008
doi: 10.1242/jeb.009175

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Sodium uptake in different life stages of crustaceans: the water flea Daphnia magna Strauss

Adalto Bianchini^1,* and Chris M. Wood²

¹ Fundação Universidade Federal do Rio Grande, Departamento de Ciências Fisiológicas, Campus Carreiros, Av. Itália s/n, 96.201-900 Rio Grande, RS, Brazil
² McMaster University, Department of Biology, 1280 Main Street West, Hamilton, ON, L8S 4K1, Canada

^* Author for correspondence (e-mail: adalto{at}octopus.furg.br)

Accepted 3 December 2007

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.

Key words: crustacean, Daphnia magna, ion transport, life stage, Na⁺ uptake

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