Bicarbonate permeability of epithelial chloride channels

Bicarbonate permeability of epithelial chloride channels has been studied using the patch-clamp technique. The experiments were performed in excised insideout oriented membrane patches from three different cultured cell types: (a) HT₂₉ colon carcinoma cell line, (b) T₈₄ colon carcinoma cell line, and (c) respiratory epithelial cells (REC) in primary culture. In all three preparations we observed outwardly rectifying chloride channels with similar conductances with 145 mmol/l NaCl solution in the pipette and in the bath (Cl⁻_pipette/ Cl⁻_bath). When Cl⁻ was replaced by HCO ₃ ⁻ in the bath (Cl⁻/HCO ₃ ⁻ ) the conductance of the channel at negative clamp voltages was reduced significantly by 40% for HT₂₉ (n=6), 39% for T₈₄ (n=7), and 38% for REC (n=6). Similarly, the zero-current potential (_VI=0) was shifted from 0 mV (Cl⁻/Cl⁻) to negative values (Cl⁻/ HCO ₃ ⁻ ) revealing permeability ratios \({{P_{{\text{Cl}}} } \mathord{\left/ {\vphantom {{P_{{\text{Cl}}} } {P_{{\text{H}}_{{\text{CO}}_{\text{3}} } } }}} \right. \kern-\nulldelimiterspace} {P_{{\text{H}}_{{\text{CO}}_{\text{3}} } } }}\) of 2.4±0.1 for HT₂₉ (n=6), 2.0±0.1 for T₈₄ (n=7), and 1.8±0.1 for REC (n=7). With NaHCO₃ as the pipette solution and NaCl in the bath, the VI=0 was positive and a \({{P_{{\text{Cl}}} } \mathord{\left/ {\vphantom {{P_{{\text{Cl}}} } {P_{{\text{H}}_{{\text{CO}}_{\text{3}} } } }}} \right. \kern-\nulldelimiterspace} {P_{{\text{H}}_{{\text{CO}}_{\text{3}} } } }}\), value of 2.3±0.1 was determined for HT₂₉ (n=5). Replacement of Cl⁻ in the bath by HCO ₃ ⁻ reduced V_I=0 to values close to 0 mV. In another series of experiments, the pipette was filled with 145 mmol/l NaCl and the bath contained 35 mmol/l NaCl to which 35 mmol/l NaHCO₃ were added. We found that neither the conductance for the inward current nor V_I=0 was changed significantly with the additon of NaHCO₃ (HT₂₉, n=6). We conclude that the HCO ₃ ⁻ permeability and HCO ₃ ⁻ conductance of these channels is about half of that for Cl⁻.