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1 Department of Physiology and Biophysics, Weill Medical College of Cornell University, New York, NY, USA
2 Department of Pharmacology and Toxicology, University of Lausanne, Lausanne, Switzerland
* To whom correspondence should be addressed. E-mail: lgpalm{at}med.cornell.edu.
Currents through epithelial Na channels were measured in the cortical collecting tubule (CCT) of mice expressing truncated 
-subunits of ENaC, reproducing one of the mutations found in human patients with Liddle's syndrome. Tubules were isolated from mice homozygous for the Liddle mutation (L/L) and from wild-type littermates (WT). Amiloride-sensitive currents (INa) from single cells were recorded under whole-cell clamp conditions. CCT's from mice kept under control conditions and fed a diet with normal levels of Na had very small I Na 's (WT: 18 ± 13 pA; L/L 22 ± 8 pA at Vm = -100 mV) which were not different in WT and L/L animals. However the L/L mice had much larger currents when the animals were fed a low-Na diet (WT: 256 ± 127 pA; L/L: 1820 ± 330 pA) or infused with aldosterone (WT: 285 ± 63 pA; L/L 1600 ± 280 pA). Currents from L/L mice were also larger when animals were pretreated with a high K diet, but not when the CCT's were stimulated in vitro with 8-CTP-cAMP. Noise analysis of amiloride-induced fluctuations in I Na showed that single channel currents at Vm = 0 mV were slightly smaller in L/L mice (WT: 0.33 pA; L/L 0.24 pA). This difference could be attributed to a decrease in driving force since I-V analysis indicated that intracellular Na was increased in the L/L animals. Analysis of spontaneous channel noise indicated that the open probability was similar in the two genotypes (WT 0.77; L/L 0.80). Thus the increase in whole-cell current is attributed to a difference in the density of conducting channels.
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