Analysis of the time course of calcium-activated chloride ''tail'' currents in rabbit portal vein smooth muscle cells

The time course of calcium-activated chloride ''tail'' currents (I-tail) in single cells of the rabbit portal vein was studied. These currents were activated by the influx of calcium through voltage-dependent calcium channels (VDCCs). At -50 mV, I-tail decayed exponentially with a time constant (tau) of 80-100 ms that was independent of amplitude and was similar to the tau of the decay of spontaneous transient inward currents (STICs; calcium-activated chloride currents). The decays of the STIC and I-tail had a similar voltage dependence between -50 and -110 mV and were similarly affected by the chloride channel blocker, niflumic acid. However, at more positive potentials (-20 to +40 mV), I-tail was sustained for the duration of the test pulse in most cells, in contrast to STICs which decayed exponentially At very positive potentials (e.g. +100 mV), when little calcium enters the cell through VDCCs, I-tail decayed exponentially. Measurement of calcium current (I-Ca) at various potentials showed that the VDCCs did not inactivate fury at potentials between -20 and +30 mV. We propose that at negative potentials the decay of I-tail is determined by slow gating of the chloride channel, but at positive potentials a sustained I-tail is produced by persistent influx of calcium through non-inactivating VDCCs.