Secretory diarrhoea reflects the activation of electrogenic Cl– secretion within the small intestine and/or colon. This involves activation of basolateral K+ channels, which maintain a favourable electrochemical gradient for Cl– exit across the apical (lumenal) membrane. At the mRNA level, KCNQ1 (KvLQT1) and KCNE3 (MiRP2) have been identified in mouse and human colon (Schroeder et al. 2000; Morton et al. 2001). When co-expressed in a heterologous system, these channels are stimulated by cAMP and inhibited by chromanol 293B. The present study sought functional evidence for KCNQ1/KCNE3 expression in human colonic crypts.
With ethics committee approval and written informed consent, biopsies were obtained during colonoscopy from the distal colon of patients who were free of mucosal disease. Intact crypts were isolated by Ca2+ chelation and stored at 4 °C for up to 24 h prior to use. Crypts were transferred to a perfusion chamber and immobilised by coating the base of the chamber with poly-L-lysine. Patch pipettes contained predominantly potassium gluconate and 0.24 mg ml-1 amphotericin. Whole-cell clamp was established within 10 min of seal formation. Data are given as means ± 1 S.E.M. and comparisons made with Student’s unpaired t test.
Whole-cell currents were activated within 5 min by addition of 10 µM forskolin to the bath solution (current at +100 mV: 123 ± 21.4 pA control, 410 ± 79.2 pA after forskolin, n = 3). This forskolin-activated current was inhibited by 100 µM chromanol 293B and 1 mM BaCl2. Prior to forskolin addition, currents were insensitive to chromanol 293B. In addition to these whole-cell recordings, small conductance channels of approximately 7 pS have been observed in cell-attached recordings with K+-rich pipette solutions.
In conclusion, small conductance KCNE3/KCNQ1 channels are expressed in the basolateral membrane of human colonic crypts. These channels are stimulated by forskolin and inhibited by chromanol 293B and Ba2+ and may be expected to play a role in the pathogenesis of secretory diarrhoea.
This work was supported by The Wellcome Trust and the MRC. Chromanol 293B was a kind gift from Dr A.E. Busch of Aventis Pharma.
All procedures accord with current local guidelines and the Declaration of Helsinki.