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The inward rectifier K+ current underlies oscillatory membrane potential behaviour in bovine pigmented ciliary epithelial cells.

1. Fresh bovine, pigmented ciliary epithelial cells possess an inward rectifier current activated by hyperpolarization. This current was investigated using whole‐cell patch‐clamp techniques. At the holding potential of ‐70 mV, and with EK (potassium equilibrium potential) set at ‐84 mV, a small outward current flowed through the inward rectifier that was sensitive to external K+, becoming more outward in 0.5 mM K+ and progressively more inward in 20 and 50 mM K+. 2. The inward rectifier showed V‐EK dependence; increasing [K+]o increased the inward conductance from 1.28 nS in 5 mM K+ to 7.42 nS in 50 mM K+. The conductance at a given V‐EK was proportional to the square root of [K+]o. 3. It was blocked by external Cs+ but replacing K+ in the pipette with Cs+ blocked only outward ion movement through the inward rectifier. Inward rectification was also blocked by Ba2+ (85% with Ki (concentration giving half‐maximal inhibition) = 3.1 x 10(‐5) M) and TEA+ (74% with Ki = 2.9 x 10(‐4) M). 4. The activation time constant was voltage dependent, decreasing from 5 ms to 0.7 ms over the voltage range ‐90 to ‐170 mV. With increasing hyperpolarization the current exhibited time‐dependent decay. The time constant for this process was voltage sensitive but the steady‐state inactivation was independent of external [K+]. 5. The current disappeared in culture within 8 days. 6. Solution flow over the cell inactivated the inward rectifier, a property that may be related to [K+]o. 7. In current clamp the cells exhibited an unstable region at a potential of around ‐70 mV. Once in this region oscillations and regenerative hyperpolarizing potentials were observed. This behaviour was eliminated by treatments that blocked (Cs+, Ba2+) or removed (0.5 mM K+) active inward rectification. 8. It is suggested that these oscillations may represent a process of cation loading, the first step in the secretion of aqueous humour.