Cl- secretion induced by bile salts. A study of the mechanism of action based on a cultured colonic epithelial cell line.

Abstract

When applied to the basolateral (serosal) side of the T84 coIonic epithelial monolayer, taurodeoxycholate caused net Clsecretion in a dose-dependent manner with a threshold effect observed at 0.2 mM. In contrast, when applied to the apical (luminal) surface, concentrations of taurodeoxycholate below 1 mM had little or no effect. Only when the concentration of taurodeoxycholate present on the apical side was 2 1 mM did apical addition results in an electrolyte transport effect. This apical effect on electrolyte transport was associated with an abrupt increase in the permeability of the monolayer. Cyclic AMP and cyclic GMP in the T84 monolayers were not increased by the bile salt, but in the presence of extracellular Ca2", free cytosolic Ca2" increased with a graded dose effect and time course that corresponded approximately to the changes in short circuit current (ISC). The results suggest that luminal bile salts at a relatively high concentration (2 1 mM) increase tight junction permeability. Once tight junction permeability increases, luminal bile salts could reach the basolateral membrane of the epithelial cells where they act to increase free cytosolic Ca2" from extracellular sources. The resulting increases in free cytosolic Ca2", rather than in cyclic nucleotides, appear to be involved in transcellular Cl-secretion.

Basolateral Ca2+‐dependent K+‐channels play a key role in Cl—secretion induced by taurodeoxycholate from colon mucosa

“…Although not directly investigated in the present model, this might also apply to the prosecretory effect of TDC. In accord with this, bile salts do not increase cAMP and cGMP colonic intracellular concentrations (Dharmsathaphorn et al, 1989). Nevertheless, in our experiments though the colon was stripped, a paracrine activity cannot be completely excluded.…”

“…A schematic representation of the mechanisms for Cl -secretion induced by TDC in proximal colon mucosa. TDC activates the Ca 2+ -dependent K + channels-rSK4-on the basolateral membrane by increasing the extracellular Ca 2+ influx (Dharmsathaphorn et al, 1989). The activation of these channels hyperpolarizes the membrane potential, provides the Na + /K + /2Cl -basolateral influx, thereby the driving force for the apical exit of Cl -ions throughout the CFTR.…”

“…Based on both in vivo and in vitro evidences, several hypotheses have been proposed, including increased mucosal permeability (Chadwick et al, 1979) and Ca 2+ influx through the apical membrane (Frizzell, 1977), as well as mediation by local neuro-humoral responses (Karlstrom et al, 1983). In T 84 colonic epithelial monolayers, TDC increased I SC and the effect was due to increased Cl -secretion (Dharmsathaphorn et al, 1989). Stimulation of electrogenic Cl -secretion induced by TDC occurs, in rabbit distal colon, via an increase in intracellular Ca 2+ concentration (Venkatasubramanian et al, 2001).…”

hSK4, a member of a novel subfamily of calcium-activated potassium channels

“…In the lung, where hSK4 also is expressed, a charybdotoxin-sensitive SK-like channel is thought to hyperpolarize airway epithelia in response to agonists that elevate intracellular Ca 2ϩ and cAMP, thereby facilitating apical Cl Ϫ secretion (35,36). In the colon, activation of Ca 2ϩ -dependent K ϩ channels again appears to be coupled to apical Cl Ϫ secretion (34,54,55). Although we did not examine colonic mRNA for the presence of hSK4, ESTs derived from colon match hSK4, and a rat homolog of hSK4 has been cloned from a colonic cDNA library (V. Rajendran, personal com-FIG.…”

Natural Bile Acids and Synthetic Analogues Modulate Large Conductance Ca2+-activated K+ (BKCa) Channel Activity in Smooth Muscle Cells

“…Previous studies that showed bile acid-induced increases in K ϩ currents were performed in nonsmooth muscle preparations (Binah et al, 1987;Kotake et al, 1989;Devor et al, 1993). In particular, an increase in a Ca 2ϩ -sensitive conductance in colonic secretory cells by taurodeoxycholic acid led Dharmsathaphorn et al (1989) to postulate an involvement of Ca 2ϩ -dependent K ϩ channels in taurodeoxycholic acid effects on secretion in these cells. These previous studies did not address the particular channel type targeted by bile acids.…”