A large-conductance chloride channel in pigmented ciliary epithelial cells activated by GTPgammaS

A large-conductance (or maxi-) chloride channel was identified in bovine pigmented ciliary epithelial (PCE) cells using inside-out excised patch clamp recording. The channel had a mean conductance of 293 pS when excised patches were bathed in symmetrical 130 mm NaCl although the conductance decreased to 209 pS when the solution bathing the cytoplasmic face of the patch contained only 33 mm NaCl. The channel was highly selective for chloride, with a PCl/PNa = 24. A flickery, reversible block was produced by the diuretic stilbene 4-acetamido-4'-isothiocyanatostilbene-2,2'-disulfonic acid (SITS), while 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS) produced a permanent block. The channel was rarely active in cell-attached patches and usually required several minutes of polarization before activity could be detected in excised patches, a process known as metagenesis. Once activated, the channel was voltage-dependent and was mainly open within the voltage range -30 to +30 mV closing when the membrane was polarized to larger values. GTPgammaS (100 microM) activated the channel with a latency of 170 sec when applied to the cytoplasmic face of patches. This activation was not reversible upon return to control solution within the duration of the experiment. We assess the available evidence and suggest a role for this channel in volume regulation.