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 muM tenidap (71 +/- 8%) and by 100 muM 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.