Intracellular ion activities in Malpighian tubule cells of Rhodnius prolixus: evaluation of Na+-K+-2Cl- cotransport across the basolateral membrane.
Intracellular ion activities (a(ion)) and basolateral membrane potential (Vbl) were measured in Malpighian tubule cells of Rhodnius prolixus using double-barrelled ion-selective microelectrodes. In saline containing 103 mmol l(-1) Na+, 6 mmol l(-1) K+ and 93 mmol l(-1) Cl-, intracellular ion activities in unstimulated upper Malpighian tubules were 21, 86 and 32 mmol l(-1), respectively. In serotonin-stimulated tubules, aCl was unchanged, whereas aNa increased to 33 mmol l(-1) and aK declined to 71 mmol l(-1). Vbl was -59 mV and -63 mV for unstimulated and stimulated tubules, respectively. Calculated electrochemical potentials (deltamuF) 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, deltamuF 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 (deltamu(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 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.
Ianowski
Juan P
JP
Department of Biology, McMaster University, 1280 Main Street West, Hamilton, Ontario, Canada L8S 4K1. ianowsjp@mcmaster.ca
Christensen
Robert J
RJ
O'Donnell
Michael J
MJ
D000818:Animals; D001464:Barium / Q000494:pharmacology; D002034:Bumetanide / Q000494:pharmacology; D002462:Cell Membrane / Q000502:physiology; D002712:Chlorides / Q000378:metabolism; D004563:Electrochemistry; D004791:Enzyme Inhibitors / Q000494:pharmacology; D007700:Kinetics; D008317:Malpighian Tubules / Q000187:drug effects / Q000502:physiology*; D008564:Membrane Potentials; D008839:Microelectrodes; D010042:Ouabain / Q000494:pharmacology; D011188:Potassium / Q000378:metabolism; D012239:Rhodnius / Q000502:physiology*; D012701:Serotonin / Q000494:pharmacology; D012964:Sodium / Q000378:metabolism; D028021:Sodium-Potassium-Chloride Symporters / Q000378:metabolism*; D013816:Thermodynamics
D002712:Chlorides; D004791:Enzyme Inhibitors; D028021:Sodium-Potassium-Chloride Symporters; D002034:Bumetanide; D001464:Barium; D012701:Serotonin; D010042:Ouabain; D012964:Sodium; D011188:Potassium