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 an d perforated-patch recording techniques. RPE cells exhibited both outw ard and inward voltage-dependent currents and had a mean membrane capa citance of 26 +/- 12 pF (SD, n = 92). The resting membrane potential a veraged -31 +/- 15 m V (n = 37), but it was as high as -60 m V in some cells. When K+ was the principal cation in the recording electrode, d epolarization-activated outward currents were apparent in 91% of cells studied. Tail current analysis revealed that the outward currents wer e primarily K+ selective. The most frequently observed outward K+ curr ent was a voltage- and time-dependent outward current (I-K) which rese mbled the delayed rectifier K+ current described in other cells. I-K w as blocked by tetraethylammonium ions (TEA) and barium (Ba2+) and redu ced by 4-aminopyridine (4-AP). In a few cells (3-4%), depolarization t o -50 m V or more negative potentials evoked an outwardly rectifying K + current (I-Kt) which showed more rapid inactivation at depolarized p otentials. Inwardly rectifying K+ current (I-KI) was also present in 4 1% of cells. I-KI was blocked by extracellular Ba2+ or Cs+ and exhibit ed time-dependent decay, due to Na+ blockade, at negative potentials. We conclude that cultured rabbit RPE cells exhibit at least three volt age-dependent K+ currents. The K+ conductances reported here may provi de conductive pathways important in maintaining ion and fluid homeosta sis in the subretinal space.