Transport properties of the human intestinal anion exchanger DRA (down-regulated in adenoma) in transfected HEK293 cells

Electroneutral NaCl absorption in the intestine is mediated by parallel Na+/H+ and Cl−/HCO3− exchange. Mutations in the down-regulated in adenoma (DRA) gene cause congenital chloride diarrhoea but the transport characteristics of human DRA have not been studied in a heterologous human expression system. A N-terminal enhanced green fluorescence protein (EGFP)-tagged human DRA construct was therefore expressed stably in HEK293 cells. Cl−/HCO3− exchange was assessed by measuring intracellular pH and intracellular Cl− using fluorescent dyes. Expression of DRA resulted in the appearance of EGFP fluorescence and DRA immunoreactivity consistent with a location in the plasma membrane and possibly structures below the plasma membrane. DRA mediated electroneutral Cl−/HCO3− exchange but OH− was not transported and SO42−/HCO3− exchange was minimal. In the presence of 5% CO2/HCO3− the apparent affinity of DRA for Cl− in transfected HEK cells was 23–36 mM, which is lower than that reported for rabbit ileal brush border membrane vesicles and for oocytes injected with human DRA. DRA was inhibited by 4 mM DIDS (45±11%), by 50 µM tenidap (71±8%) and by 100 µM glibenclamide (59±22% inhibition of HCO3− transport and 79±3% inhibition of Cl− transport). The effects of DIDS and tenidap were not additive to those of glibenclamide.