Potassium currents in cultured rabbit retinal pigment epithelial cells

Membrane potential and ionic currents were studied in cultured rabbit retinal pigment epithelial (RPE) cells using whole-cell patch clamp and perforated-patch recording techniques. RPE cells exhibited both outward and inward voltage-dependent currents and had a mean membrane capacitance of 26±12 pF (sd, n=92). The resting membrane potential averaged -31±15 mV (n=37), but it was as high as -60 mV in some cells. When K⁺ was the principal cation in the recording electrode, depolarization-activated outward currents were apparent in 91% of cells studied. Tail current analysis revealed that the outward currents were primarily K⁺ selective. The most frequently observed outward K⁺ current was a voltage- and time-dependent outward current (I_K) which resembled the delayed rectifier K⁺ current described in other cells. I_K was blocked by tetraethylammonium ions (TEA) and barium (Ba²⁺) and reduced by 4-aminopyridine (4-AP). In a few cells (3-4%), depolarization to -50 mV or more negative potentials evoked an outwardly rectifying K⁺ current (I_Kt) which showed more rapid inactivation at depolarized potentials. Inwardly rectifying K⁺ current (I_KI) was also present in 41% of cells. I_KI was blocked by extracellular Ba²⁺ or Cs⁺ and exhibited time-dependent decay, due to Na⁺ blockade, at negative potentials. We conclude that cultured rabbit RPE cells exhibit at least three voltage-dependent K⁺ currents. The K⁺ conductances reported here may provide conductive pathways important in maintaining ion and fluid homeostasis in the subretinal space.