Molecular and functional studies of electrogenic Na(+) transport in the distal colon and rectum of young and elderly subjects

Background: Human distal nephron and distal colon both exhibit mineralocorticoid sensitive electrogenic Na(+) absorption and make significant contributions to Na(+) homeostasis. Na(+) resorption in the distal nephron diminishes with age but it is unclear whether a similar change occurs in the distal colon. Aims: To evaluate the effect of age on expression of apical Na(+) channels and basolateral Na(+), K(+)-ATPase, and on the responsiveness of electrogenic Na(+) absorption to mineralocorticoid stimulation in human distal colon and rectum. Materials and methods: Mucosal biopsies were obtained from healthy sigmoid colon and proximal rectum in “young” (aged 20–40 years) and “old” (aged 70 years or over) patients during routine colonoscopy/flexible sigmoidoscopy. Na(+) channel subunits and Na(+), K(+)-ATPase isoforms were studied at the mRNA level by in situ hybridisation and northern blotting, and at the protein level by immunocytochemistry and western blotting. The mineralocorticoid responsiveness of electrogenic Na(+) absorption was evaluated in the two groups by measuring amiloride sensitive electrical potential difference (PD) in the proximal rectum before and 24 hours after oral administration of 1 mg of fludrocortisone. Results: Na(+) channel subunit and Na(+), K(+)-ATPase isoform expression at the level of mRNA and protein was similar in “young” and “old” patients. Both basal and the fludrocortisone stimulated amiloride sensitive rectal PDs were similar in the two groups. Conclusions: In contrast with the distal nephron, mineralocorticoid sensitive electrogenic Na(+) absorption in the human distal colon does not diminish with age, and may be particularly important in maintaining Na(+) homeostasis in the elderly

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) α-subunit levels but increases the expression of the β- and γ-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 α-subunit mRNA throughout the colon in control and Na+-depleted animals. By contrast, Na+ depletion substantially enhanced β-subunit mRNA expression in the distal colon, but had no effect on the low expression levels of β-subunit mRNA in the proximal colon. Expression of the γ-subunit, evaluated by PCR, was also restricted to the distal colon of Na+-depleted animals. Western analyses demonstrated similar levels of α-subunit protein in the proximal and distal colon of both groups of animals, whereas β-subunit and γ-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 β- and γ-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 α-subunit mRNA and protein) because of its inability to enhance β- and γ-subunit expression in this segment