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The Journal of Experimental Biology 205, 1645-1655 (2002)
© 2002 The Company of Biologists Limited

Intracellular ion activities in Malpighian tubule cells of Rhodnius prolixus: evaluation of Na⁺-K⁺-2Cl^- cotransport across the basolateral membrane

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

^* e-mail: ianowsjp{at}mcmaster.ca

Accepted 18 March 2002

Intracellular ion activities (a_ion) and basolateral membrane potential (V_bl) were measured in Malpighian tubule cells of Rhodnius prolixus using double-barrelled ion-selective microelectrodes. In saline containing 103mmoll^-1 Na⁺, 6mmoll^-1 K⁺ and 93mmoll^-1 Cl^-, intracellular ion activities in unstimulated upper Malpighian tubules were 21, 86 and 32mmoll^-1, respectively. In serotonin-stimulated tubules, a_Cl was unchanged, whereas a_Na increased to 33mmoll^-1 and a_K declined to 71mmoll^-1. V_bl was -59mV and -63mV for unstimulated and stimulated tubules, respectively. Calculated electrochemical potentials (µ/F) favour passive movement of Na⁺ into the cell and passive movement of Cl^- out of the cell in both unstimulated and serotonin-stimulated tubules. Passive movement of K⁺ out of the cell is favoured in unstimulated tubules. In stimulated tubules, µ/F for K⁺ is close to 0 mV.

The thermodynamic feasibilities of Na⁺-K⁺-2Cl^-, Na⁺-Cl^- and K⁺-Cl^- cotransporters were evaluated by calculating the net electrochemical potential (µ_net/F) for each transporter. Our results show that a Na⁺-K⁺-2Cl^- or a Na⁺-Cl^- cotransporter but not a K⁺-Cl^- cotransporter would permit the movement of ions into the cell in stimulated tubules. The effects of Ba²⁺ and ouabain on V_bl and rates of fluid and ion secretion show that net entry of K⁺ through ion channels or the Na⁺/K⁺-ATPase can be ruled out in stimulated tubules. Maintenance of intracellular Cl^- activity was dependent upon the presence of both Na⁺ and K⁺ in the bathing saline. Bumetanide reduced the fluxes of both Na⁺ and K⁺. Taken together, the results support the involvement of a basolateral Na⁺-K⁺-2Cl^- cotransporter in serotonin-stimulated fluid secretion by Rhodnius prolixus Malpighian tubules.

Key words: Rhodnius prolixus, Malpighian tubule, ion-selective microelectrode, intracellular ion activity, electrochemical potential, ion transport, Na⁺-K⁺-2Cl^- cotransporter

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