Evidence for independent Cl‐ and HCO3‐ secretion and involvement of an apical Na(+)‐HCO3‐ cotransporter in cultured rat epididymal epithelia

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Abstract
Electrogenic chloride and bicarbonate secretion by cultured rat epididymal epithelia was studied using the short‐circuit current (ISC) technique. When incubated in normal solution, 8‐(4‐chlorophenylthio)‐adenosine 3',5'‐cyclic monophosphate (cpt‐cAMP) caused a rise in the ISC, which was attributable to Cl‐ and HCO3‐ secretion. Cl‐ secretion was found to contribute to the initial transient phase, whereas HCO3‐ secretion contributed to the sustained phase of the response. HCO3‐ secretion involves a basolaterally placed Na(+)‐H+ exchanger and apical anion channel, most probably the cystic fibrosis transmembrane conductance regulator (CFTR). There is also evidence that an apical electrogenic Na(+)‐HCO3‐ cotransporter is involved in HCO3‐ exit. CFTR accounted for 70% of HCO3‐ secretion, while the Na(+)‐HCO3‐ cotransporter accounted for 30%. The possibility that the cotransporter may serve as an alternative pathway for HCO3‐ secretion in cystic fibrosis is discussed.

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