Distinct K+ conductive pathways are required for Cl- and K+ secretion across distal colonic epithelium.

Secretion of Cl(-) and K(+) in the colonic epithelium operates through a cellular mechanism requiring K(+) channels in the basolateral and apical membranes. Transepithelial current [short-circuit current (I(sc))] and conductance (G(t)) were measured for isolated distal colonic mucosa during secretory activation by epinephrine (Epi) or PGE(2) and synergistically by PGE(2) and carbachol (PGE(2) + CCh). TRAM-34 at 0.5 microM, an inhibitor of K(Ca)3.1 (IK, Kcnn4) K(+) channels (H. Wulff, M. J. Miller, W. Hänsel, S. Grissmer, M. D. Cahalan, and K. G. Chandy. Proc Natl Acad Sci USA 97: 8151-8156, 2000), did not alter secretory I(sc) or G(t) in guinea pig or rat colon. The presence of K(Ca)3.1 in the mucosa was confirmed by immunoblot and immunofluorescence detection. At 100 microM, TRAM-34 inhibited I(sc) and G(t) activated by Epi ( approximately 4%), PGE(2) ( approximately 30%) and PGE(2) + CCh ( approximately 60%). The IC(50) of 4.0 microM implicated involvement of K(+) channels other than K(Ca)3.1. The secretory responses augmented by the K(+) channel opener 1-EBIO were inhibited only at a high concentration of TRAM-34, suggesting further that K(Ca)3.1 was not involved. Sensitivity of the synergistic response (PGE(2) + CCh) to a high concentration TRAM-34 supported a requirement for multiple K(+) conductive pathways in secretion. Clofilium (100 microM), a quaternary ammonium, inhibited Cl(-) secretory I(sc) and G(t) activated by PGE(2) ( approximately 20%) but not K(+) secretion activated by Epi. Thus Cl(-) secretion activated by physiological secretagogues occurred without apparent activity of K(Ca)3.1 channels but was dependent on other types of K(+) channels sensitive to high concentrations of TRAM-34 and/or clofilium.