Early aldosterone effect in distal colon by transcriptional regulation of ENaC subunits

Aldosterone-induced sodium absorption is mediated by the epithelial Na+ cha nnel (ENaC). It is thought that the "early effect" is not based on genomic regulation of ENaC expression, because ENaC subunit transcription was repor ted to start later than Na+ transport. We investigated electrogenic Na+ abs orption (J(Na)) and, in identical tissues, mRNA expression of ENaC subunits in early (EDC) and late (LDC) distal colon of the rat. In both segments, 8 -h in vitro incubation with 3 nM aldosterone enhanced expression of beta- a nd gamma-ENaC mRNA and induced J(Na-) J(Na) was 10 times higher in LDC than in EDC. alpha-ENaC mRNA was unchanged in EDC, whereas it decreased in LDC. In LDC, beta- and gamma-ENaC mRNA was induced Ih after aldosterone additio n, whereas J(Na) became apparent >1 h later. Downregulation of alpha-ENaC m RNA did not take part in acute regulation because it started after a lag ti me of 3 h. Time correlation of beta- and gamma-ENaC induction and J(Na) Sti mulation suggests that the early aldosterone effect on Na+ absorption in di stal colon is caused by transcriptional upregulation of beta- and gamma-ENa C expression.