Calmodulin Antagonist W-7 Enhances Intermediate Conductance Ca2+- Sensitive Basolateral Potassium Channel (IKCa) Activity in Human Colonic Crypts

Abstract

Intermediate conductance potassium (IK_Ca) channels are exquisitively Ca²⁺ sensitive, intracellular Ca²⁺ regulating channel activity by complexing with calmodulin (CaM), which is bound to the cytosolic carboxyl tail. Although CaM antagonists might be expected to decrease IK_Ca channel activity, the effect of W-7 in human T lymphocytes are conflicting. We therefore evaluated the effect of W-7 on basolateral IK_Ca channels in human colonic crypt cells. Intact crypts obtained from normal human colonic biopsies by Ca²⁺ chelation were used for patch clamp studies of basolateral IK_Ca channels in the cell-attached configuration. IK_Ca channel activity was studied when the bath Ca²⁺ concentration was changed from 1.2 mmol/L to 100 μmol/L and back to 1.2 mmol/L, as well as from 100 μmol/L to 1.2 mmol/L and back to 100 μmol/L, both in the absence and presence of 25 μmol/L W-7. Decreasing bath Ca²⁺ from 1.2 mmol/L to 100 μmol/L decreased IK_Ca channel activity reversibly in the absence of W-7, whereas there was a uniformly high level of channel activity at both bath Ca²⁺ concentrations in the presence of W-7. In separate experiments, increasing bath Ca²⁺ from 100 μmol/L to 1.2 mmol/L increased IK_Ca channel activity reversibly in the absence of W-7, whereas there was again a uniformly high level of channel activity at both bath Ca²⁺ concentrations in the presence of W-7. We, therefore, propose that W-7 has a specific stimulatory effect on basolateral IK_Ca channel activity, despite its ability to inhibit Ca²⁺/CaM-mediated, IK_Ca channel-dependent Cl^- secretion in human colonic epithelial cells.

Keywords: Calmodulin; Colon; Potassium channels; W-7.