A nonselective high conductance channel in bovine pigmented ciliary epithelial cells

An ion channel activated by hyperpolarization was identified in excised patches of bovine pigmented ciliary epithelial cells using the single channel patch clamp technique. In symmetrical NaGluconate, the channel had a maximum conductance of 285 pS. The channel was characterized by frequent flickery transitions between the fully open and closed levels. The channel did not discriminate very clearly between anions and cations; when the cytoplasmic face of excised patches was bathed in a dilute NaCl solution, the channel had a chloride-to-sodium permeability ratio (PCl/PNa) of 1.3. However, the channel showed a small anion selectivity (PCl/PNa = 3.7) when bathed in a concentrated NaCl solution. Gd3+ blocked the channel reversibly. Channel kinetics were characterised by slow (approximately min) voltage-dependent activation and inactivation rate constants. The channel was most active in the range-60 to -140 mV and showed a peak at -120 mV. A similar time- and voltage-dependent activation was also observed in cell-attached recordings. In conclusion, hyperpolarization of pigmented ciliary epithelial cell membrane patches activated a large conductance, nonselective ion channel. This combination of nonselectivity and hyperpolarizing activation is consistent with the involvement of this channel in ion loading from the blood into pigmented ciliary epithelial cells-the first phase in the secretion of aqueous humor.