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Regulation of a hyperpolarization‐activated chloride current in murine respiratory ciliated cells

1 The properties of a hyperpolarization‐activated Cl− current (Ihyp‐act) in murine ciliated respiratory cells have been studied using whole cell patch clamping. 2 The current‐voltage relationship was inwardly rectifying which was due to voltage‐dependent gating of the channel. 3 Inward current was markedly sensitive to the extracellular Cl− concentration, an effect that was not related to changes in transmembrane Cl− gradient. Decreasing extracellular Cl− concentration to 6 mM caused a 70 % reduction in inward current with the dose‐response relationship exhibiting a Hill coefficient of ≈2·0 and an IC50 of 29 mM. 4 External anion replacement gave a selectivity sequence of Br− > I− > Cl− > gluconate = aspartate. The more permeant halides significantly increased current density while the less permeant anions decreased current density, indicating that an extracellular anion is important for channel activity. 5 The conductance was unaffected by exposure to anisotonic pipette solutions or to increases in intracellular cAMP; however, current density was reduced dose dependently by increases in intracellular calcium concentration from 0·1 to 0·5 μM. These results indicate that Ihyp‐act is unlikely to be involved in either volume regulation or cAMP/Ca2+‐stimulated fluid secretion. 6 Decreasing extracellular pH to 5·0 irreversibly inhibited Ihyp‐act. However, the current was fully active over the pH range 5·4‐9·0 making it unlikely that it is modulated by extracellular pH under physiological conditions. 7 We speculate that Ihyp‐act may have a role in basal Cl− absorption, acting as a Cl− sensor to maintain optimal volume and composition of airway surface liquid.