Na channel expression and activity in the medullary collecting duct of rat kidney

The expression and activity of epithelial Na+ channels (ENaC) in the medullary collecting duct of the rat kidney were examined using a combination of whole cell patch-clamp measurements of amiloride-sensitive currents (I-Na) in split-open tubules and Western blot analysis of alpha-, beta-, and gamma-ENaC proteins. In the outer medullary collecting duct, amiloride-sensitive currents were undetectable in principal cells from control animals but were robust when rats were treated with aldosterone (I-Na = 960 +/- 160 pA/cell) or fed a low-Na diet (I-Na = 440 +/- 120 pA/cell). In both cases, the currents were similar to those measured in principal cells of the cortical collecting duct from the same animals. In the inner medullary collecting duct, 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 1 wk, similar changes in alpha- and gamma-ENaC occurred in all three regions of the kidney; the amounts of full-length as well as putative cleaved alpha-ENaC protein increased, and the fraction of gamma-ENaC protein in the cleaved state increased at the expense of the full-length protein. The appearance of a presumably fully glycosylated form of beta-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.