Can ENaC regulate ICF as well as ECF volume? Focus on "Osmotic pressure regulates αβγ-rENaC expressed in Xenopus oocytes".

IN THE CURRENT article in focus, Ji et al. (Ref. 5, see p. C1182 in this issue) present evidence that osmotic pressure can acutely regulate activity of the amiloridesensitive epithelial Na1 channel (ENaC) expressed in oocytes. Given the restricted tissue distribution of ENaC, is it plausible that ENaC could play an important role in cell volume regulation in response to anisosmotic stimuli in vivo? The kidney regulates extracellular fluid (ECF) volume by adjusting Na1 transport rate along the kidney tubules and ECF osmolarity by adjusting permeability of the renal tubules to water. Close control of ECF volume and osmolarity requires simultaneous regulation of Na1 and water transporters resident in the cortical collecting tubules and collecting ducts, and homeostasis is accomplished by excretion of urine highly variable in volume and osmolarity. A consequence is that the apical surfaces of the epithelial cells in the distal nephron are bathed in renal tubule fluid that is likewise highly variable (Fig. 1). In other words, as the cells regulate whole body ECF volume and osmolarity, they are presented with the significant challenge of maintaining their own intracellular fluid (ICF) volume and osmolarity. In the presence of antidiuretic hormone (ADH), water permeability is high along collecting tubules and ducts and tubule fluid osmolarity can increase up to 1,200 mosM. In the absence of ADH, this region becomes impermeable to water and osmolarity falls to as low as 50 mosM as salts are reabsorbed. These ADH-stimulated changes in permeability can occur quite rapidly, and the renal cells in this region must quickly adjust to variable tonicity. At the molecular level, Na1 is reabsorbed along the nephron by a number of apical transporters expressed in a region-specific pattern (Fig. 1). Up to the distal tubule, the osmotic environment of any given region of the renal tubule cells is fairly constant, and long-term adjustments to the chronic hyperosmolarity of the medullary loop of Henle and chronic hyposmolarity of the ascending loop and distal tubule are in place to maintain cell volume. Beyond the distal tubule, there are acute fluctuations in tubule fluid osmolarity and in the same region there is fine adjustment of the ECF volume by the regulating activity of the apical amiloridesensitive ENaC in the cortical collecting tubules and collecting ducts. For example, ENaC activity can be rapidly increased by aldosterone and ADH in this region (Fig. 1), which is associated with increased phosphorylation of ENaC subunits (3, 4, 10) and results in increased ECF volume. Additional evidence that ENaC plays a key role in controlling ECF volume is that mutations in the channel that cause increased activity lead to hypertension, whereas inactivating mutations are associated with hypotension (reviewed in Refs. 3 and 4). Because of the hand-in-hand association of rapid Na1 and water transport regulation and consequent exposure to apical anisosmotic fluid in the collecting tubules and ducts, it would be propitious if ENaC could play a role in rapidly sensing and maintaining cell volume in its host cell. Besides kidney, ENaC is located in other epithelia (including colon, sweat and salivary glands, amphibian skin, and bladder; Ref. 4) where Na1 and water transport are highly regulated, and these tissues must also face the challenge of maintaining ICF volume and osmolarity in a fluctuating milieu. Cell shape is critical for optimal function of alveolar type I cells, which also express ENaC (4); cell swelling would lengthen the pathway for oxygen diffusion and gas exchange. The ENaC a-, b-, and g-subunits share significant homology and membrane topology features with the mechanosensitive degenerins of Caenorhabditis elegans (3, 4). This has stimulated investigators to look for evidence of ENaC mechanosensitivity, i.e., response to membrane stretch (1, 2, 5, 7, 8). Amiloride-sensitive stretch activation has been demonstrated in B lymphocytes (1), bovine a-ENaC or abg rat ENaC (rENaC) in planar lipid bilayers (2), and reconstituted a-ENaC from osteoblasts (7). The work by Ji et al. (5) goes beyond the issue of stretch activation to test the hypothesis that amiloride-sensitive ENaCs play a role in cell volume regulation. Their choice of Xenopus laevis oocytes injected with abg-rENaC takes advantage of the apparent lack of background regulatory volume decrease (RVD) in response to hypotonic media and lack of regulatory volume increase (RVI) in response to hypertonic media (5, 6). A similar recent study that also used injected oocytes (6) asked whether expression of an anion exchanger (AE2) could function-