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First published online December 15, 2004
Journal of Experimental Biology 208, 93-104 (2005)
Published by The Company of Biologists 2005
doi: 10.1242/jeb.01374

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Ion-selective microelectrode analysis of salicylate transport by the Malpighian tubules and gut of Drosophila melanogaster

Michael J. O'Donnell^* and Mark R. Rheault

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

^* Author for correspondence (e-mail: odonnell{at}mcmaster.ca)

Accepted 7 October 2004

Transport of the organic anion salicylate by the Malpighian tubules and gut of larval and adult fruit flies was studied using two salicylate-selective microelectrode methods. The first method combined the high selectivity of tridodecylmethylammonium-based electrodes for salicylate with the self-referencing ion-selective microelectrode technique for non-invasive spatial and temporal analysis of salicylate flux. Measurements with this technique revealed secretion of salicylate across the main and distal segments of the Malpighian tubule as well as the midgut, ileum and rectum. The second method used a salicylate-selective microelectrode to measure the concentration of salicylate in fluid droplets secreted by isolated Drosophila Malpighian tubules set up in a Ramsay secretion assay. Transepithelial salicylate flux was calculated as the product of fluid secretion rate and secreted fluid salicylate concentration. Measurements with this method revealed that salicylate transport was active and saturable; the kinetic parameters J_max and K_t were 2.72 pmol min^-1 tubule^-1 and 0.046 mmol l^-1, respectively. Measurements of transepithelial salicylate flux determined by both microelectrode methods were in good agreement. Transepithelial flux measurements measured by microelectrodes were also validated by comparing them with measurements of radiolabelled salicylate levels in secreted droplets. Salicylate concentrations in haemolymph samples were measured with salicylate-selective microelectrodes after injection of salicylate into the haemocoel or after insects were fed salicylate-rich diets. The rate of salicylate secretion by Malpighian tubules in vitro was sufficient to account for the measured rate of decline of salicylate concentration in the haemolymph in vivo.

Key words: salicylate, ion-selective microelectrode, organic anion transport, Malpighian tubule

This article has been cited by other articles:

				E. Ruiz-Sanchez and M. J. O'Donnell Effects of chronic exposure to dietary salicylate on elimination and renal excretion of salicylate by Drosophila melanogaster larvae J. Exp. Biol., July 15, 2007; 210(14): 2464 - 2471. [Abstract] [Full Text] [PDF]