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
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
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Fig. 1. Sodium influx (µequiv g–1 wet mass
h–1) in Daphnia magna as a function of external
sodium concentration, pH (4, 6 or 8) and calcium concentration (A, 0 mmol
l–1; B, 0.5 mmol l–1; C, 1 mmol
l–1). Each plotted point represents the mean ±
S.E.M. of 5–6 daphnids exposed under experimental conditions
described in Materials and methods.
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Fig. 2. Effect of external pH (4, 6 or 8) and calcium (0, 0.5 or 1 mmol
l–1) on Daphnia magna sodium transport capacity (A;
Jmax, µequiv g–1 wet mass
h–1) and affinity (B; Km, µmol
l–1). Plotted points represent parameters calculated directly
from the plots illustrated in Fig.
1A–C, using SigmaPlot (version 8.0.2; SPSS, Inc.). Missing
data points (*) were those that did not conform to
Michaelis–Menten kinetics, and were thus excluded. Bars sharing
lowercase letters are not significantly different compared to other pH
treatments within each calcium concentrations (i.e. effect of pH), whereas
bars sharing uppercase letters are not significantly different compared to
similar pH treatments at different calcium concentrations (i.e. effect of
calcium). Statistical significance (P<0.05) was calculated as
described in Materials and methods. Data points with asterisks were excluded
from statistical comparison.
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Fig. 3. Whole body sodium (mequiv kg–1 wet mass) in Daphnia
magna as a function of external sodium concentration, pH (4, 6 or 8) and
calcium (A, 0 mmol l–1; B, 0.5 mmol l–1; C,
1 mmol l–1). Each plotted point represents the mean ±
S.E.M. of 5–6 daphnids exposed under experimental conditions
described in Materials and methods. Data points sharing letters are not
significantly different (P<0.05) from similar points within each
sodium concentration, as determined by two-way ANOVA.
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Fig. 4. Effect of external calcium concentration (0–5000 µmol
l–1 calcium gluconate) on sodium influx (µequiv
g–1 wet mass h–1) in Daphnia magna
at two different sodium concentrations (A, 50 µmol l–1; B,
1000 µmol l–1), and two different sodium salts (sodium
chloride, white circles; sodium gluconate, black circles). Each data point
represents the mean of 5–6 individuals. *Statistical
significance (P<0.05) was determined by one-way ANOVA, and
represents differences between the plotted point and the control (0 calcium).
There were no significant differences in the calcium dependence of sodium
influx between sodium chloride and sodium gluconate.
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Fig. 5. Effect of amiloride (0–10 000 µmol l–1) on sodium
influx (µequiv g–1 wet mass h–1) in
Daphnia magna at two different sodium concentrations (50 or 300
µmol l–1). *Statistical significance
(P<0.05) was determined by one way ANOVA, and represents
differences between the plotted point and the control (0 amiloride). The data
represented in A were replotted in the Dixon plot (B). The
Ki of amiloride inhibition of sodium influx was calculated
as the inverse of the x-value of the intersection point of the two lines. Some
data points at low amiloride concentrations were excluded for clarity, and the
10 000 µmol l–1 amiloride concentration was excluded from
the Dixon plot owing to maximal inhibition being attained at 5000 µmol
l–1 amiloride.
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© The Company of Biologists Ltd 2005
