Altered cryptal expression of luminal potassium (BK) channels in ulcerative colitis

Decreased sodium (Na(+)), chloride (Cl(-)), and water absorption, and increased potassium (K(+)) secretion, contribute to the pathogenesis of diarrhoea in ulcerative colitis. The cellular abnormalities underlying decreased Na(+) and Cl(-) absorption are becoming clearer, but the mechanism of increased K(+) secretion is unknown. Human colon is normally a K(+) secretory epithelium, making it likely that K(+) channels are expressed in the luminal (apical) membrane. Based on the assumption that these K(+) channels resembled the high conductance luminal K(+) (BK) channels previously identified in rat colon, we used molecular and patch clamp recording techniques to evaluate BK channel expression in normal and inflamed human colon, and the distribution and characteristics of these channels in normal colon. In normal colon, BK channel alpha-subunit protein was immunolocalized to surface cells and upper crypt cells. By contrast, in ulcerative colitis, although BK channel alpha-subunit protein expression was unchanged in surface cells, it extended along the entire crypt irrespective of whether the disease was active or quiescent. BK channel alpha-subunit protein and mRNA expression (evaluated by western blotting and real-time PCR, respectively) were similar in the normal ascending and sigmoid colon. Of the four possible beta-subunits (beta(1-4)), the beta(1)- and beta(3)-subunits were dominant. Voltage-dependent, barium-inhibitable, luminal K(+) channels with a unitary conductance of 214 pS were identified at low abundance in the luminal membrane of surface cells around the openings of sigmoid colonic crypts. We conclude that increased faecal K(+) losses in ulcerative colitis, and possibly other diseases associated with altered colonic K(+) transport, may reflect wider expression of luminal BK channels along the crypt axis.