Anomalous effect of anthracene‐9‐carboxylic acid on calcium‐activated chloride currents in rabbit pulmonary artery smooth muscle cells

Ca 2+ ‐activated Cl − currents (I Cl(Ca) ) evoked by K + ‐free pipette solutions containing 500 n M Ca 2+ were recorded in rabbit pulmonary artery smooth muscle cells. A voltage step protocol in which the cells were stepped to +70 mV and then to −80 mV produced outward and inward Cl − currents respectively that exhibited distinctive voltage‐ and time‐dependent kinetics that remained consistent for the recording period. Application of the Cl − channel inhibitor anthracene‐9‐carboxylic acid (A‐9‐C, 500 μ M ), produced a small inhibition of the maximum outward Cl − current at +70 mV (21±10%) but augmented the amplitude of the instantaneous inward relaxation at −80 mV by 321±34% ( n =12). The current recorded in the absence and presence of A‐9‐C reversed at the theoretical Cl − equilibrium potential and the reversal potential was shifted by about −40 mV upon replacement of external chloride ion by the more permeant anion thiocyanate. Currents in the absence and presence of A‐9‐C were similarly affected by 100 μ M niflumic acid. Augmentation of the inward current at −80 mV by A‐9‐C required prior depolarization, i.e. A‐9‐C did not simply activate a Cl − current at negative membrane potentials. Moreover the degree of augmentation was independent of the internal Ca 2+ for concentrations between 100 n M and 1 μ M Ca 2+ . The data from the present study confirm previous observations that the inhibitory effect of Cl − channel blockers is modified when [Ca 2+ ] i is maintained at higher than normal resting concentrations.British Journal of Pharmacology (2003) 138 , 31–38. doi: 10.1038/sj.bjp.0705000