Modulation of Chloride Secretion by Bile Acids in Airway Epithelial Cells.
Bile acids are often present in the lower airways of people with cystic fibrosis and other respiratory distress diseases, probably resulting from the aspiration of gastroesophageal refluxate. The effects of bile acids on airway epithelium ion transport function have not yet been investigated. The effects of the unconjugated secondary bile acids, DCA and UDCA, plus the taurine conjugate of DCA, TDCA, on basal ion transport and carbachol (CCh)- or forskolin (Fsk)- induced Cl' secretion was investigated in Calu-3 airway epithelial cells grown in an air-liquid interface and mounted in Ussing chambers. Electrogenic transepithelial ion transport was measured as short-circuit current (lsc)- These results show that acute (5 min) apical treatment with DCA had no effect on basal lsc but decreased CCh- stimulated lsc by 43%. UDCA had no acute effect on lsc in Calu-3 cells. In contrast, 24 hr apical treatment with DCA or UDCA increased CCh- stimulated lsc by 36% and 65% respectively and 24 hr UDCA treatment also attenuated subsequent Fsk responses by 17%. Furthermore, basolateral TDCA treatment of Calu-3 cells stimulated basal lsc by 39% but had no effect on the lsc responses induced by CCh or Fsk treatment. The lsc responses to TDCA were abolished in Cl'-free Krebs solution indicating that TDCA modulates Cl'secretion. TDCA had no chronic effect on lsc in Calu-3 cells.
Acute treatment with TDCA produced a 32% increase in CFTR lsc that was abolished by pre-treatment with CFTRjnhi72- It was also found that TDCA increases Cl' secretion through calcium-activated chloride (CaCC) channels by 18%. In addition, when Na+/K+ ATPase generated currents were measured in Ussing chambers, acute treatment with TDCA increased Na+/K+ pump activity by 13%, while pre-treatment with ouabain eliminated this effect. Acute treatment with TDCA resulted in a rapid 88% increase in intracellular calcium mobilization, which was abolished in Ca2+ -free buffer indicating that TDCA induced Ca2+ influx into the cell. In addition, TDCA increased basal cAMP levels and PKA activation. Preliminary data from TGR5 knockdown studies suggest that TGR5 is the membrane receptor involved in TDCA induced Cl' secretion. In summary, this thesis shows for the first time that low concentrations of bile acids can modulate Cl' secretion in airway epithelial cells and this effect is dependent upon both the duration and sidedness of exposure to the bile acid.