The colonic epithelium absorbs and secretes electrolytes and water. Ion and water absorption occurs primarily in surface cells, whereas crypt cells perform secretion. Ion transport in distal colon is regulated by aldosterone, which stimulates both Na+ absorption and K+ secretion. The electrogenic Na+ absorption is mediated by epithelial Na+ channel (ENaC) in surface cells. Previously, we identified the large conductance Ca2+-activated K+ channel, KCa1.1 or big potassium (BK) channel, as the only relevant K+ secretory pathway in mouse distal colon. The exact localisation of KCa1.1 channels along the crypt axis is, however, still controversial. The aim of this project was to further define the localisation of the KCa1.1 channel in mouse distal colonic epithelium. Through quantification of mRNA extracted from micro-dissected surface and crypt cells, we confirmed that Na+/K+/2Cl− (NKCC1) is expressed primarily in the crypts and γ-ENaC primarily in the surface cells. The KCa1.1 α-subunit mRNA was like NKCC1, mainly expressed in the crypts. The crypt to surface expression pattern of the channels and transporters was not altered when plasma aldosterone was elevated. The mRNA levels for NKCC1, γ-ENaC and KCa1.1 α-subunit were, however, under these circumstances substantially augmented (KCa1.1 α-subunit, twofold; NKCC1, twofold and ENaC, tenfold). Functionally, we show that ENaC-mediated Na+ absorption and BK channel-mediated K+ secretion are two independent processes. These findings show that KCa1.1-mediated K+ secretion mainly occurs in the crypts of the murine distal colon. This is in agreement with the general model of ion secretion being preferentially located to the crypt and not surface enterocytes.
