Background Human colon may secrete substantial amounts of water secondary to chloride (Cl⁻) and/or potassium (K⁺) secretion in a variety of diarrhoeal diseases. Ion secretion occurs via Cl⁻ and K⁺ channels, which are generally assumed to be co-located in the colonocyte apical membrane, although their exact cellular sites remain unclear.

Objective  To investigate the location of apical Cl⁻ (CFTR) and apical K⁺ (large conductance; BK) channels within human colonic epithelium.

Design Whole-cell patch clamp recordings were obtained from intact human colonic crypts. Specific blockers of K⁺ channels and CFTR identified different types of K⁺ channel and CFTR under resting conditions and after stimulating intracellular cAMP with forskolin. The BK channel β₃-subunit was localised by immunostaining.

Results Two types of crypt cells were identified. One (73% of cells) had whole-cell currents dominated by intermediate conductance (IK) K⁺ channels under resting conditions, which developed large CFTR-mediated currents in response to increasing intracellular cAMP. The other (27% of cells) had resting currents dominated by BK channels inhibited by the BK channel blocker penitrem A, but insensitive to both forskolin and the IK channel blocker clotrimazole. Immunostaining showed co-localisation of the BK channel β₃-subunit and the goblet cell marker, MUC2.

Conclusions In human colon, Cl⁻ secretion originates from the dominant population of colonocytes expressing apical CFTR, whereas K⁺ secretion is derived from a smaller population of goblet cells expressing apical BK channels. These findings provide new insights into the pathophysiology of secretory diarrhoea and should be taken into account during the development of anti-diarrhoeal drugs.