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1 Department of Physiology and Biophysics, Weill Medical College of Cornell University, New York, NY, USA
* To whom correspondence should be addressed. E-mail: lgpalm{at}med.cornell.edu.
Epithelial Na channels were investigated using patch-clamp techniques in connecting tubule (CNT) segments isolated from rat kidney. Cell-attached patches with Li+ in the patch pipette contained channels with conductances for inward currents of 13-16 pS and slow opening and closing kinetics, similar to properties of Na channels in the cortical collecting tubule (CCT). Macroscopic amiloride-sensitive currents (INa) were also observed under whole-cell clamp conditions. These currents were undetectable in cells from control rats but were large when the animals were infused with aldosterone (1380 ± 340 pA/cell at a holding potential of -100 mV) or fed a high K diet (670 ± 260 pA/cell) for one week. Under both of these conditions currents in cells of the CNT were 2-4 fold larger than those of the CCT's of the same animals. In aldosterone-treated animals, currents in cells of the initial collecting tubule (iCT) were intermediate, such that the relative magnitude of INa was CNT > iCT> CCT. Quantitative analysis of the results suggests that the maximal capacity of the CNT to reabsorb Na could be as high as 18 µmoles/min, or about 10% of the filtered load of Na. This capacity is about 10-times higher than that of the CCT.
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