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First published online September 15, 2004
Journal of Experimental Biology 207, 3707-3716 (2004)
Published by The Company of Biologists 2004
doi: 10.1242/jeb.01203

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Na⁺ competes with K⁺ in bumetanide-sensitive transport by Malpighian tubules of Rhodnius prolixus

Juan P. Ianowski, Robert J. Christensen and Michael J. O'Donnell^*

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

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

Accepted 15 July 2004

We examined the effects of bathing saline Na⁺/K⁺ ratio, bumetanide and hydrochlorothiazide on fluid and ion transport by serotonin-stimulated Malpighian tubules of Rhodnius prolixus. Previous pharmacological and electrophysiological studies indicate that a bumetanide-sensitive Na⁺/K⁺/2Cl^– cotransporter is the primary route for basolateral ion entry into the cell during fluid secretion. The goal of this study was to resolve the apparent conflict between relatively high secretion rates by tubules bathed in K⁺-free saline and the evidence that Na⁺/K⁺/2Cl^– cotransporters described in other systems have an absolute requirement for all three ions for translocation. Our measurements of fluid secretion rate, ion fluxes and electrophysiological responses to serotonin show that fluid secretion in K⁺-free saline is bumetanide sensitive and hydrochlorothiazide insensitive. Dose–response curves of secretion rate versus bumetanide concentration were identical for tubules bathed in K⁺-free and control saline with IC₅₀ values of 2.6x10^–6 mmol l^–1 and 2.9x10^–6 mmol l^–1, respectively. Double-reciprocal plots of K⁺ flux versus bathing saline K⁺ concentration showed that increasing Na⁺ concentration in the bathing fluid increased K_t but had no effect on J_max, consistent with competitive inhibition of K⁺ transport by Na⁺. We propose that the competition between Na⁺ and K⁺ for transport by the bumetanide-sensitive transporter is part of an autonomous mechanism by which Malpighian tubules regulate haemolymph K⁺ concentration.

Key words: ion transport, Na⁺/K⁺/2Cl^– cotransporter, stoichiometry, K⁺ homeostasis

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