Bicarbonate permeability of the outwardly rectifying anion channel

Single anion-selective channels have been studied in cultured human epithelial cells using the patch-clamp technique. Three cell types were used as models for different anion transport systems: (i) PANC-1, a cell line derived from the pancreatic duct, (ii) T84, a Cl-secreting colonic cell line, and (iii) primary cultures of sweat duct epithelium. Outwardly rectifying anion-selective channels were observed in all three preparations and were indistinguishable with respect to conductance, selectivity and gating. Striking similarities between HCO3- and Cl-secreting epithelia, and the high density of outward rectifiers in pancreatic cells prompted us to study HCO3 permeation through this channel. HCO3 permeability was significant when channels were bathed in symmetrical 150 mM HCO3 solutions, Cl-HCO3 mixtures, and under bi-ionic conditions with outwardly and inwardly directed HCO3 gradients. Permeability ratios (PHCO3/PCl) estimated from bi-ionic reversal potentials ranged from 0.50 to 0.64, although conductance ratios greater than 1.2 were observed with high extracellular pH. Chloride did not inhibit HCO3 permeation noticeably but rather had a small stimulatory effect when present on the opposite side of the membrane. The prevalence of outward rectifiers in PANC-1 and their permeability to bicarbonate suggests the channel may have a dual role in HCO3 secretion; to allow Cl recycling at the apical membrane and to mediate some of the HCO3 flux. Defective modulation of this channel in cystic fibrosis might provide a common basis for dysfunction in epithelia having very different anion transport properties (e.g., HCO3 secretion, Cl secretion and Cl absorption.