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First published online March 8, 2005
Journal of Experimental Biology 208, 951-959 (2005)
Published by The Company of Biologists 2005
doi: 10.1242/jeb.01426

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Physiological characterisation of a pH- and calcium-dependent sodium uptake mechanism in the freshwater crustacean, Daphnia magna

Chris N. Glover^* and Chris M. Wood

Department of Biology, McMaster University, Hamilton, Ontario, Canada

^* Author for correspondence at National Institute for Nutrition and Seafood Research, Postboks 2029 Nordnes, 5817 Bergen, Norway (e-mail: chris.glover{at}nifes.no)

Accepted 1 December 2004

Daphnia are highly sensitive to sodium metabolism disruption caused by aquatic acidification and ionoregulatory toxicants, due to their finely balanced ion homeostasis. Nine different water chemistries of varying pH (4, 6 and 8) and calcium concentration (0, 0.5 and 1 mmol l^–1) were used to delineate the mechanism of sodium influx in Daphnia magna. Lowering water pH severely inhibited sodium influx when calcium concentration was high, but transport kinetic analysis revealed a stimulated sodium influx capacity (J_max) when calcium was absent. At low pH increasing water calcium levels decreased J_max and raised K_m (decreased sodium influx affinity), while at high pH the opposite pattern was observed (elevated J_max and reduced K_m). These effects on sodium influx were mirrored by changes in whole body sodium levels. Further examination of the effect of calcium on sodium influx showed a severe inhibition of sodium uptake by 100 µmol l^–1 calcium gluconate at both low (50 µmol l^–1) and high (1000 µmol l^–1) sodium concentrations. At high sodium concentrations, stimulated sodium influx was noted with elevated calcium levels. These results, in addition to data showing amiloride inhibition of sodium influx (K_i=180 µmol l^–1), suggest a mechanism of sodium influx in Daphnia magna that involves the electrogenic 2Na⁺/1H⁺ exchanger.

Key words: acid precipitation, soft water, hardness, osmoregulation, invertebrate, Daphnia magna

This article has been cited by other articles:

				A. Bianchini and C. M. Wood Sodium uptake in different life stages of crustaceans: the water flea Daphnia magna Strauss J. Exp. Biol., February 15, 2008; 211(4): 539 - 547. [Abstract] [Full Text] [PDF]