Calcium-activated potassium current in cultured rabbit retinal pigment epithelial cells

Calcium-activated potassium current was studied in cultured rabbit retinal pigment epithelial (RPE) cells using whole-cell and single channel patch-clamp recording techniques. When K⁺ was the principal cation in the electrode, depolarizing voltage steps from a holding potential of -60 mV activated outwardly rectifying current. Outward K⁺ current was increased by the Ca²⁺ ionophore ionomycin and reduced when the extracellular Ca²⁺ concentration was decreased from 2.5 mM to 100 nM in the presence of ionomycin. Outward K⁺ current recorded in the presence of ionomycin was blocked by iberiotoxin and by charybdotoxin. Single channel recording from cell-attached and excised membrane patches revealed a large conductance Ca²⁺ activated K⁺ (K((Ca))) channel. Identification of K((Ca)) channels was based on: 1) the voltage-dependence of channel opening; 2) the large unitary conductance (> 200 pS with symmetrical 130 mM K⁺); 3) the dependence of the reversal potential on the K⁺ gradient; and 4) increased channel opening after exposure of the cytosolic surface of excised membrane patches to elevated Ca²⁺. These results demonstrate that Ca²⁺-activated K⁺ channels are present in rabbit RPE cells and may play an essential role in the regulation of membrane potential and ion transport.