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WATER AND ELECTROLYTE HOMEOSTASIS

Characterization of tetraethylammonium uptake across the basolateral membrane of the Drosophila Malpighian (renal) tubule

Department of Biology, McMaster University, Hamilton, Ontario, Canada

Submitted 15 February 2005 ; accepted in final form 22 April 2005

Basolateral transport of the prototypical type I organic cation tetraethylammonium (TEA) by the Malpighian tubules of Drosophila melanogaster was studied using measurements of basolateral membrane potential (V_bl) and uptake of [¹⁴C]-labeled TEA. TEA uptake was metabolically dependent and saturable (maximal rate of mediated TEA uptake by all potential transport processes, reflecting the total transport capacity of the membrane, 0.87 pmol·tubule^–1·min^–1; concentration of TEA at 0.5 of the maximal rate of TEA uptake value, 24 µM). TEA uptake in Malpighian tubules was inhibited by a number of type I (e.g., cimetidine, quinine, and TEA) and type II (e.g., verapamil) organic cations and was dependent on V_bl. TEA uptake was reduced in response to conditions that depolarized V_bl (high-K⁺ saline, Na⁺-free saline, NaCN) and increased in conditions that hyperpolarized V_bl (low-K⁺ saline). Addition of TEA to the saline bathing Malpighian tubules rapidly depolarized the V_bl, indicating that TEA uptake was electrogenic. Blockade of K⁺ channels with Ba²⁺ did not block effects of TEA on V_bl or TEA uptake indicating that TEA uptake does not occur through K⁺ channels. This is the first study to provide physiological evidence for an electrogenic carrier-mediated basolateral organic cation transport mechanism in insect Malpighian tubules. Our results also suggest that the mechanism of basolateral TEA uptake by Malpighian tubules is distinct from that found in vertebrate renal tubules.

Drosophila melanogaster; Malpighian tubule; organic cation transport; comparative renal physiology