Segmental variability of ENaC subunit expression in rat colon during dietary sodium depletion

Abstract

In rat distal colon, aldosterone has little effect on Na(+) channel (ENaC) alpha-subunit levels but increases the expression of the beta- and gamma-subunits and stimulates electrogenic Na(+) transport. By contrast, the molecular basis of aldosterone's inability to stimulate electrogenic Na(+) transport in the proximal colon is unclear. We therefore compared the effects of hyperaldosteronism secondary to 10 days dietary Na(+) depletion on ENaC subunit expression in rat proximal and distal colon. Northern analyses revealed appreciable and similar levels of alpha-subunit mRNA throughout the colon in control and Na(+)-depleted animals. By contrast, Na(+) depletion substantially enhanced beta-subunit mRNA expression in the distal colon, but had no effect on the low expression levels of beta-subunit mRNA in the proximal colon. Expression of the gamma-subunit, evaluated by PCR, was also restricted to the distal colon of Na(+)-depleted animals. Western analyses demonstrated similar levels of alpha-subunit protein in the proximal and distal colon of both groups of animals, whereas beta-subunit and gamma-subunit proteins were detected solely or predominantly in the distal colon of the Na(+)-depleted animals. Immunocytochemistry confirmed that significant levels of all three subunit proteins only occurred in the apical membrane of surface cells in the distal colon of Na(+)-depleted animals. Our findings are consistent with previous studies demonstrating that aldosterone stimulates electrogenic Na(+) transport in rat distal colon by increasing the expression of beta- and gamma-subunit mRNA and protein, and thus the amount of functional heteromeric ENaC protein in the apical domain. They also show that aldosterone is incapable of stimulating electrogenic Na(+) transport in rat proximal colon (despite the presence of alpha-subunit mRNA and protein) because of its inability to enhance beta- and gamma-subunit expression in this segment.

High Salt Intake Down-Regulates Colonic Mineralocorticoid Receptors, Epithelial Sodium Channels and 11β-Hydroxysteroid Dehydrogenase Type 2

“…The differential response of the ENaC-α vs. ENaC-β and ENaC-γ is in line with the literature [36], [37], [38]. In the distal colon from rats, low dietary NaCl or high concentrations of mineralocorticoids increased the β- and γ-subunits whereas the α-subunit remained unchanged [38], [39], [40], [41]. Here we did not detect a change in Sgk-1 expression as a function of salt intake.…”

Intestinal Mineralocorticoid Receptor Contributes to Epithelial Sodium Channel–Mediated Intestinal Sodium Absorption and Blood Pressure Regulation

“…ENaC is a heterotrimer of α, β, and γ subunits, all of which are essential for channel functionality 30. The main target of MR in the kidneys is α‐ENaC, whereas in the intestine, β‐ and γ‐ENaC are the main targets 31, 32, 33. The reduction in β‐ and γ‐ENaC but not α‐ENaC expression in the colons of IEC‐MR‐KO mice was consistent with these previous findings.…”

Keratins modulate colonocyte electrolyte transport via protein mistargeting

“…ENaCγ was particularly strong apically in K8−/− crypt colonocytes ( Fig. 7 B, h), as compared with previously described ENaC isoforms on the apical membrane of rat colonocytes ( Greig et al, 2002 ). Therefore, the abnormal Na/Cl transport in K8−/− can be attributed to a generalized protein mistargeting that affects several ion transporters, and possibly by increased expression of the AE1/2 anion exchanger.…”