The Role Of Bicarbonate In Regulating Transepithelial Ion Transport In Pig Tracheal Epithelia

Airway epithelia utilize Na Cl transport for regulating salt and water movement across the epithelial layer,whereas transepithelial HCO₃^-transport is used for maintaining intra-and extracellular p H and associated function in bacterial killing and mucus maturation.However,it remains unclear whether HCO₃^-transport may regulate transepithelial NaCl transport for modulating epithelial function.This study investigated whether HCO₃^-transport may have a cross-regulation of transepithelial salt transport in primarily cultured pig tracheal epithelia.To test this hypothesis,I first used the Ussing Chamber method to examine the effect of HCO₃^--free medium on transepithelial ion transports of cultured epithelia.The data show that HCO₃^--free medium did not alter the short-circuit current（Isc）at the basal condition.However,the reduction in Isc by the epithelial Na⁺channel（ENaC）inhibitor amiloride was diminished after the treatment.These data suggest that the channel activity of ENa C was reduced by the treatment of HCO₃^--free medium.By contrast,after the cystic fibrosis transmembrane conductance regulator（CFTR）Cl^-channel was fully activated by drugs forskolin and IBMX,the following combined reduction in Isc by specific inhibitors Gly H-101 for CFTR,bumetanide for Na⁺/K⁺/2Cl^-cotransporter and DPC for other Cl^–-dependent transporters was evidently increased.The data suggest that CFTR activity was enhanced by HCO₃^--free medium.The real-time PCR data indicate that the m RNA expression of ENa C-α,-β,-γsubunits might be reduced in the HCO₃^--free group.Other key transporters like CFTR,Na⁺/K⁺/2Cl^-cotransporter and Na⁺/K⁺ATPase showed no apparent alterations in the m RNA expression by HCO₃^--free medium.To test whether the extracellular signal-regulated kinase 1 and 2（ERK）-mediated phosphorylation on ENa C may contribute to the reduced amiloride-sensitive Isc by HCO₃^--free medium,the ERK specific inhibitor U0126 was used to treat cultured epithelia but the data indicate that the drug alone caused a strong inhibition on Isc.In summary,my data revealed that withdrawing HCO₃^-from culture medium may alter transepithelial Na⁺and Cl^-transport of cultured pig tracheal epithelia.Such the cross-regulation of salt transport by HCO₃^-in epithelial cells may suggest an important role of HCO₃^-in the regulation of epithelial physiological function and the pathogenesis of epithelial disorders.