First published online January 16, 2009
Journal of Experimental Biology 212, 363-372 (2009)
Published by The Company of Biologists 2009
doi: 10.1242/jeb.023739
Too much of a good thing: how insects cope with excess ions or toxins in the diet
M. J. O'Donnell
Department of Biology, McMaster University, 1280 Main Street West,
Hamilton, Ontario, Canada L8S 4K1
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Fig. 1. Schematic diagrams showing (A) the excretory system in Rhodnius
prolixus and current models of (B) Na+, K+,
Cl– and H2O secretion across the upper Malpighian
tubule (UMT) and (C) K+ and Cl– reabsorption
across the lower Malpighian tubule (LMT). Pi, inorganic
phosphate.
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Fig. 3. Non-invasive measurement of ion fluxes by the scanning ion-selective
electrode technique (SIET). (A) The technique exploits gradients in ion
activity within the unstirred layer that are created by ion transport across
cells or epithelia. In this example, secretion of tetraethylammonium (TEA)
across the epithelium in the direction of the arrows reduces the concentration
of TEA close to the epithelial surface relative to that farther away. The size
of the labels and the density of the shading correspond to the concentration
of TEA. (B) Schematic diagram of equipment used for SIET measurements. The
isolated tissue and the ion-selective microelectrode (ISME) are observed
through a microscope equipped with a CCD camera. A computer-controlled motion
control system drives an orthogonal (X, Y, Z) array
of stepper motors which move the amplifier headstage and attached ISME to
sites along the tissue and then at two points orthogonal to the tissue
surface, as indicated in A. Voltage differences between the two limits of
excursion are recorded by the data acquisition system. Voltage gradients at
different sites can be overlaid as vectors on an image of the tissue captured
by the frame grabber connected to the CCD camera (as shown in
Fig. 5A).
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Fig. 6. Plots of TEA influx as a function of bathing saline TEA concentration for
(A) the LMT, (B) the main segment and (C) the posterior midgut. Each point is
the mean ± s.e.m. of N=4–7 preparations. Values for
Jmax and Kt were determined by
non-linear regression analysis as described in Rheault and O'Donnell
(Rheault and O'Donnell, 2004 ),
from which this figure is taken.
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Fig. 7. Effects of chronic exposure of Drosophila larvae to dietary
salicylate on (A) fluid secretion rate, (B) salicylate concentration in the
secreted fluid and (C) transepithelial flux of salicylate across the main
segment of isolated Malpighian tubules set up in the Ramsay assay. Each point
represents the mean ± s.e.m. of 7–10 tubules. Solid and broken
lines indicate the control and experimental group, respectively. Experimental
larvae were raised for 10 days on a 10 mmol l–1
salicylate-enriched diet. Control larvae were raised on a salicylate-free
diet. Figure replotted from Ruiz-Sanchez and O'Donnell
(Ruiz-Sanchez and O'Donnell,
2007b).
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© The Company of Biologists Ltd 2009
) operational amplifier and
recorded on a PC-based data acquisition system. TEA concentration in the
secreted droplets is calculated from the voltage difference between the
secreted droplet and the calibration droplets, as described by Rheault and
O'Donnell (Rheault and O'Donnell,
2004
10%
polyvinyl chloride (PVC) in tetrahydrofuran to avoid capillary rise of the
paraffin oil into the silanized microelectrode. (B) Photograph showing 20
Drosophila tubules set up in a Ramsay assay in a 55 mm diameter Petri
dish. One of the 20 µl bathing droplets is indicated by the white arrow.
Each tubule is secured to a black minutien pin anchored in the Sylgard to the
right of each bathing saline droplet. Three secreted droplets collected at 30
min intervals are shown within the red ellipse. Reference and TEA-selective
microelectrodes (green arrows) are positioned in a bathing saline droplet.
V; µV) between the two limits of
microelectrode excursion by subtracting the voltage at the outer limit of the
excursion from that measured at the inner limit. The length of each arrow
corresponds to the magnitude of TEA influx. (B) TEA influx is plotted as a
function of distance from the ureter along lower, main and distal segments of
the MT. Each point is the mean ± s.e.m. of four tubules. Influx of TEA
reduces TEA concentration in the unstirred layer adjacent to the surface of
the tissue, and the corresponding voltage difference is therefore negative.
Distance 0 on the abscissa corresponds to the junction of the ureter and the
LMT. Both the differential signal recorded by the TEA-selective microelectrode
(right ordinate) and the calculated TEA influx (left ordinate) are shown.
Figure is replotted from Rheault and O'Donnell
(Rheault and O'Donnell,
2004