A LOW-CONDUCTANCE CHLORIDE CHANNEL IN THE BASOLATERAL MEMBRANES OF THE NONPIGMENTED CILIARY EPITHELIUM OF THE RABBIT EYE

Purpose. Chloride efflux plays an important role in aqueous humor prod uction. Chloride currents have been described in bovine non-pigmented ciliary epithelium (NPE), in transformed cultured human NPE, and in vo lume-activated chloride channels described in the latter it is the bas olateral membranes of the NPE of ocular ciliary processes that compris e the exit pathway for the process of aqueous secretion performed by t he double syncytial layer of ciliary epithelium, however. Therefore, w e studied both cell-attached, and, excised, inside-out patches from ba solateral membranes of the NPE. Methods. Cell-attached and cell-free e xcised patches were formed from the basolateral membranes of NPE and s ingle channel currents recorded with a Dagan 3900A patch clamp amplifi er. Results. A low conductance channel of 14 pS was observed and recor ded in 30% of cell attached patches and in 35% of excised inside out p atches under symmetrical conditions (160 mM chloride). This channel di splayed a nearly linear current-voltage relationship, with an open pro bability that was not voltage-dependent. The channel was chloride-sele ctive: N-methyl-D-glucamine (NMDG) used as cation did not alter the cu rrent profile nor the reversal potential. Further, with inside-out pat ches, the reversal potential was close to zero (0.3 +/- 0.5 mV (10) in symmetrical (160 mM) chloride, but shifted to -32.3 +/- 0.5 mV (5) wh en the concentration of chloride in the bathing solution was reduced t o 40 mM while the recording pipette was held at 160 mM, This value app roaches the theoretical equilibrium potential of chloride for these co nditions. The channel anion permeability sequence was: I- > NO3- great er than or equal to Br- > Cl- > gluconate(-) approximate to aspartate( -). Three different chloride-channel blockers inhibited the channel ac tivity. Conclusion. A low conductance channel, selective for chloride, and, modulated by beta adrenergic and VIP stimulation, based on its s ensitivity to exogenously added cAMP, ATP and the catalytic subunit of PKA, is present in the exit basolateral membranes of rabbit NPE and c ontributes to the process of aqueous humor formation.