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First published online June 16, 2004
Journal of Experimental Biology 207, 2599-2609 (2004)
Published by The Company of Biologists 2004
doi: 10.1242/jeb.01058

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Basolateral ion transport mechanisms during fluid secretion by Drosophila Malpighian tubules: Na⁺ recycling, Na⁺:K⁺:2Cl^– cotransport and Cl^– conductance

Juan P. Ianowski and Michael J. O'Donnell^*

Department of Biology, McMaster University, 1280 Main Street West, Hamilton, Ontario, Canada, L8S 4K1

View larger version (13K): [in a new window]   Fig. 1. Recordings of basolateral membrane potential (top traces) and intracellular activity (lower traces) of (A) K+ and (B) Cl– in unstimulated tubules. Basolateral membrane potential and ion activity were measured simultaneously using double-barrelled ion-selective microelectrodes (ISMEs). In this and subsequent figures, impalement is indicated by the upward-pointing arrows and the removal of electrode from the cell is indicated by downward-pointing arrows.

View larger version (11K): [in a new window]   Fig. 2. Effects of 10–4 mol l–1 bumetanide on (A) fluid secretion, (B) K+ flux and (C) Na+ flux for tubules after exposure to ouabain. After exposure to 10–4 mol l–1 ouabain for 30 min, bumetanide was added to Malpighian tubules in the experimental group (filled bars) and the vehicle (0.1% ethanol) was added to the control group (open bars). Data are expressed as mean ± S.E.M. Asterisks indicate significant differences between flux before and after addition of bumetanide (paired t-test, P<0.05, N=11).

View larger version (16K): [in a new window]   Fig. 3. Recordings showing the effect of a reduction of bathing saline K+ concentration from 20 to 2 mmol l–1 (A), Cl– from 158.5 to 15.85 mmol l–1 (B) and Na+ from 132.5 to 13.25 mmol l–1 (C) on Vbl before and after addition of 6 mmol l–1 Ba2+. The tubules were exposed to the different conditions for the periods indicated by the horizontal bars. The spikes in the voltage measurements prior to treatment with Ba2+ were artefacts associated with movement of the experimenter.

View larger version (9K): [in a new window]   Fig. 4. Recordings showing the effect of 6 mmol l–1 Ba2+ on (A) Vbl and (B) intracellular K+ activity. The tubules were exposed to the control saline and to saline containing Ba2+ for the periods indicated by the horizontal bars.

View larger version (15K): [in a new window]   Fig. 5. Recordings showing the effect on Vbl of addition of 6 mmol l–1 Ba2+ before and after exposure to (A) 10–4 mol l–1 ouabain, (B) K+-free saline and (C) Cl–-free saline. The tubules were exposed to the different conditions for the periods indicated by the horizontal bars.

View larger version (21K): [in a new window]   Fig. 6. Schematic diagram indicating proposed ion transport systems involved in fluid secretion by the principal cells of the main segment of the Malpighian tubules of Drosophila melanogaster.