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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 (aion) and basolateral membrane potential (Vbl) 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, aCl was unchanged, whereas aNa increased to 33mmoll-1 and aK declined to 71mmoll-1. Vbl was -59mV and -63mV for unstimulated and stimulated tubules, respectively. Calculated electrochemical potentials ({Delta}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, {Delta}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 ({Delta}µnet/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 Vbl 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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