Direct inhibitory effect of nicardipine on basolateral K+ channels in human colonic crypts.

The most abundant basolateral K+ channels in human colonic crypt cells have a low conductance (23 pS), respond to increases in intracellular Ca2+ and cAMP, and have been implicated in intestinal electrogenic Cl- secretion. The effect of nicardipine on the activity of these K+ channels was examined by patch-clamp recording in the cell-attached and excised inside-out configurations from the basolateral membrane of single crypts isolated from biopsied samples of human distal colon. During cell-attached recordings, addition of 2 micromol/l nicardipine to crypts pretreated with 200 micromol/l dibutyryl cAMP decreased single-channel open probability by 87%, but in parallel studies nicardipine had no effect on the intracellular Ca2+ concentration. Using inside-out patches from crypts pretreated with dibutyryl cAMP (bathed in 1.2 mmol/l Ca2+), the addition of increasing concentrations of nicardipine (200 nmol/l, 2 micromol/l and 20 micromol/l) decreased single-channel open probability in a concentration-dependent manner (IC50 0.47 micromol/l). In additional experiments using stripped rat distal colonic mucosa mounted in conventional Ussing chambers, serosal addition of nicardipine at increasing concentrations (ranging from 200 nmol/l to 20 micromol/l) produced a concentration-dependent inhibition of dibutyryl-cAMP-stimulated electrogenic Cl- secretion (IC50 2 micromol/l). Taken together, these results indicate that nicardipine has a direct inhibitory action on 23-pS basolateral K+ channels in human intestinal crypt cells, which is likely to decrease cAMP-stimulated electrogenic Cl- secretion. These basolateral K+ channels may provide a focal point for the development of new strategies in the treatment of secretory diarrhoeal diseases.