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1 Physiology and Biophysics, Weill Medical College of Cornell U., New York, New York, United States
2 Department of Physiology, Weill Medical College of Cornell University , 1300 York Avenue, New York, New York, 10021, United States; Physiology and Biophysics, Weill Medical College of Cornell U., New York, New York, United States; Department of Physiology, Weill Medical College of Cornell University, New York, New York, United States
* To whom correspondence should be addressed. E-mail: lgpalm{at}med.cornell.edu.
The expression and activity of epithelial Na+ channels in the medullary collecting duct of the rat kidney was examined using a combination of whole-cell patch clamp measurements of amiloride-sensitive currents (INa) in split-open tubules and western-blot analysis of 
, 
and 
ENaC proteins. In the outer medullary collecting duct (OMCD) amiloride-sensitive currents were undetectable in principal cells from control animals but were robust when rats were treated with aldosterone (INa = 960 ± 160 pA/cell ) or fed a low-Na diet (INa = 440 ± 120 pA/cell). In both cases the currents were similar to those measured in principal cells of the CCD from the same animals. In the IMCD, currents were much lower, averaging 120 ± 20 pA/cell in aldosterone-treated rats. Immunoblots showed that all three ENaC subunits were expressed in the cortex, outer medulla and inner medulla of the rat kidney. When rats were fed a low-Na diet for one week similar changes in and ENaC occurred in all three regions of the kidney; the amounts of full-length as well as putative cleaved ENaC protein increased, and the fraction of ENaC protein in the cleaved state increased at the expense of the full-length protein. The appearance of a presumably fully glycosylated form of ENaC in Na-depleted animals was observed mainly in the outer and inner medulla. These findings suggest that the capability of hormone-regulated, channel-mediated Na reabsorption by the nephron extends at least into the outer medullary collecting duct.
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