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First published online April 8, 2004
Journal of Experimental Biology 207, 1655-1663 (2004)
Published by The Company of Biologists 2004
doi: 10.1242/jeb.00932

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Mechanisms of K⁺ transport across basolateral membranes of principal cells in Malpighian tubules of the yellow fever mosquito, Aedes aegypti

Brett N. Scott, Ming-Jiun Yu, Lenora W. Lee and Klaus W. Beyenbach^*

Department of Biomedical Sciences, Cornell University, Ithaca, NY 14853, USA

View larger version (22K): [in a new window]   Fig. 1. Transepithelial fluid secretion in isolated Malpighian tubules of female yellow fever mosquitoes studied by the method of Ramsay (1953). Cumulative secreted volume is plotted against time first under control conditions and then in the presence of (A) Ba2+ and (B) Ba2+ plus bumetanide. After the usual initial 30 min control period, fluid secreted by the tubules was removed for analysis with an electron microprobe, and 5 mmol l–1 BaCl2 (A) or 5 mmol l–1 BaCl2 plus 100 µmol l–1 bumetanide (B) was added to the peritubular Ringer solution. Fluid secreted under experimental conditions was collected at the end of the experiment, 25–70 min after introducing the agent of interest to the peritubular Ringer bath. The slope of regression lines yields the flow rate from the open end of the tubule, which equals the rate of transepithelial fluid secretion. Each tubule is used as its own control. Values are means ± S.E.M.

View larger version (16K): [in a new window]   Fig. 2. Single and combined effects of barium (Ba; 5 mmol l–1) and bumetanide (Bu; 100 µmol l–1) on transepithelial ion secretion in isolated Malpighian tubules of the yellow fever mosquito. After the usual initial 30 min control period, fluid secreted by the tubules was removed for analysis with an electron microprobe, and barium and/or bumetanide was added to the peritubular Ringer solution. Fluid secreted under experimental conditions was collected at the end of the experiment, 25–70 min after introducing the agent of interest to the peritubular Ringer bath. Data are expressed as % of control. N=7 for C and Ba; N=6 for Bu and Ba+Bu. For numeric data, see Table 1. Asterisks indicate significant differences (P<0.05). Effects of bumetanide alone are previously published data (Hegarty et al., 1991).

View larger version (53K): [in a new window]   Fig. 3. Models of transepithelial NaCl and KCl secretion in Malpighian tubules of Aedes aegypti. The cations Na+ and K+ take a transcellular pathway and Cl– takes a paracellular shunt pathway. Transcellular and paracellular pathways are electrically coupled, forming an intraepithelial current loop. The current generator is the V-type H+-ATPase located in the apical membrane of principal cells (Beyenbach, 2001). Current across the apical membrane is carried by H+. Current returning to the cytoplasmic face of the V-type H+-ATPase is carried by Cl– ions passing from the hemolymph to the tubule lumen through the paracellular pathway, by K+ and Na+ entering the cell across the basolateral membrane and by Cl– leaving the cell through Cl– channels (Yu et al., 2003). Barium blocks basolateral membrane K+ channels, thereby increasing the resistance of the basolateral membrane (Rbl) and decreasing the loop current. The net effect is the inhibition of transepithelial NaCl, KCl and fluid secretion. Bumetanide blocks Na+/K+/2Cl– cotransport across the basolateral membrane, leaving electroconductive pathways largely intact. As a result, intraepithelial current flow is not affected and rates of transepithelial cation, anion and fluid secretion remain unchanged. However, rates of transepithelial K+ secretion decrease and Na+ secretion increase because one major pathway for K+ entry is blocked, leaving three other Na+ entry pathways open. Vs, rate of fluid secretion; E, electromotive force; V, voltage; R, resistance; a, apical membrane; bl, basolateral membrane; sh, epithelial shunt; t, transepithelial.