Segmental variability of ENaC subunit expression in rat colon during dietary sodium depletion.
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.
Department of Medicine (University of Manchester) Hope Hospital, Salford, Manchester, UK.
E R
ER
Greig
E H
EH
Baker
T
T
Mathialahan
R P
RP
Boot-Handford
G I
GI
Sandle
D000818:Animals; D015152:Blotting, Northern; D015153:Blotting, Western; D003106:Colon / Q000378:metabolism*; D004039:Diet, Sodium-Restricted; D053503:Epithelial Sodium Channels; D005786:Gene Expression Regulation / Q000502:physiology; D006929:Hyperaldosteronism / Q000378:metabolism; D007150:Immunohistochemistry; D008297:Male; D016133:Polymerase Chain Reaction; D021122:Protein Subunits / Q000096:biosynthesis / Q000235:genetics; D012333:RNA, Messenger / Q000032:analysis; D051381:Rats; D017207:Rats, Sprague-Dawley; D015222:Sodium Channels / Q000096:biosynthesis* / Q000235:genetics*
D053503:Epithelial Sodium Channels; D021122:Protein Subunits; D012333:RNA, Messenger; D015222:Sodium Channels