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First published online May 13, 2004
Journal of Experimental Biology 207, 2173-2184 (2004)
Published by The Company of Biologists 2004
doi: 10.1242/jeb.01003
Organic cation transport by Malpighian tubules of Drosophila melanogaster: application of two novel electrophysiological methods
Department of Biology, McMaster University, 1280 Main Street West, Hamilton, Canada L8S 4K1
* Author for correspondence (e-mail: rheaulmr{at}mcmaster.ca)
Accepted 29 March 2004
Transport of the prototypical organic cation tetraethylammonium (TEA) by
the Malpighian tubules, ureters and gut of Drosophila melanogaster
was studied using two novel electrophysiological techniques. Both techniques
exploited the high selectivity of the cation exchanger potassium
tetra-p-chlorophenylborate for tetraalkylammonium compounds relative
to inorganic cations such as K+. In the first technique, TEA fluxes
were measured using a non-invasive self-referencing TEA-selective
microelectrode positioned in the unstirred layer near the surface of each
tissue. TEA fluxes from bath to lumen as large as 6 pmol cm–2
s–1 were measured across the lower (reabsorptive) segment of
the Malpighian tubule and the ureter bathed in saline containing 0.1 mmol
l–1 TEA. Corresponding bath-to-lumen fluxes across the
secretory main segment of the Malpighian tubule and the posterior midgut were
1 pmol cm–2 s–1. TEA transport by the
lower Malpighian tubule was enhanced by hyperpolarization of the basolateral
membrane potential and was inhibited by cimetidine, quinidine, vinblastine and
verapamil. In the second technique, TEA concentration was measured using a
TEA-selective microelectrode positioned in droplets of fluid secreted by
Malpighian tubules set up in saline droplets under oil in a Ramsay assay.
Results from the Ramsay assay confirmed the dominant role of the lower
Malpighian tubule in net transepithelial secretion of TEA and inhibition of
TEA transport by cimetidine. Kinetic parameters (Jmax and
Kt) were determined using both approaches.
Key words: organic cation transport, tetraethylammonium, ion-selective microelectrode, Malpighian tubule, Drosophila melanogaster
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