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

Abstract

1. Ca 2+-activated Cl - currents (I Cl(Ca)) evoked by K +-free pipette solutions containing 500 nM 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. 2. Application of the Cl - channel inhibitor anthracene-9-carboxylic acid (A-9-C, 500 μM), 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). 3. 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 μM niflumic acid. 4. 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 nM and 1 μM Ca 2+. 5. 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.