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Novel aspects of the transport of organic anions by the malpighian tubules of Drosophila melanogaster

Version 2 2024-06-18, 06:10
Version 1 2019-07-18, 15:18
journal contribution
posted on 2024-06-18, 06:10 authored by SM Linton, MJ O'Donnell
Para-aminohippuric acid (PAH) is a negatively charged organic ion that can pass across the epithelium of Malpighian tubules. Its mode of transport was studied in Malpighian tubules of Drosophila melanogaster. PAH transport was an active process, with a Km of 2.74 mmoll-1 and a Vmax of 88.8 pmolmin-1. Tubules had a low passive permeability to PAH, but PAH transport rates (832 nmol min-1mm2) and concentrative ability ([PAH]secretedfluid:[PAH]bath=81.2) were the highest measured to date for insects. Competition experiments indicated that there were two organic anion transporters, one that transports carboxylate compounds, such as PAH and fluorescein, and another that transports sulphonates, such as amaranth and Indigo Carmine. PAH transport appears to be maximal in vivo because the rate of transport by isolated tubules is not increased when these are challenged with cyclic AMP, cyclic GMP, leucokinin I or staurosporine. Basolateral PAH transport was inhibited by ouabain and dependent on the Na+ gradient. The Malpighian tubules appeared not to possess an organic acid/α-keto acid exchanger because PAH accumulation was not affected by low concentrations (100μmoll-1) of (α-keto acids (α-ketoglutarate, glutarate, citrate and succinate) or the activity of phosphokinase C. PAH transport may be directly coupled to the Na+ gradient, perhaps via Na+/organic acid cotransport. Fluorescence microscopy showed that transport of the carboxylate fluorescein was confined to the principal cells of the main (secretory) segment and all the cells of the lower (reabsorptive) segment. Organic anions were transported across the cytoplasm of the principal cells both by diffusion and in vesicles. The accumulation of punctate fluorescence in the lumen is consistent with exocytosis of the cytoplasmic vesicles. Apical PAH transport was independent of the apical membrane potential and may not occur by an electrodiffusive mechanism.

History

Journal

Journal of experimental biology

Volume

203

Pagination

3575-3584

Location

Cambridge, Eng.

ISSN

0022-0949

Language

eng

Publication classification

C1.1 Refereed article in a scholarly journal

Copyright notice

2000, The Company of Biologists

Issue

23

Publisher

Company of Biologists

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