PROPERTIES OF THE INWARDLY RECTIFYING K-PIGMENT EPITHELIUM( CONDUCTANCE IN THE TOAD RETINAL)

1. An inwardly rectifying K+ current was analysed in isolated toad ret inal pigment epithelial (RPE) cells using the perforated-patch clamp t echnique. 2. The zero-current potential (V-o) of RPE cells averaged -7 1 mV when the extracellular K+ concentration ([K+](o)) was 2mM. Increa sing [K+](o) from 0.5 to 5mM shifted V-o by +43 mV, indicating a relat ive K+ conductance (T-K) of 0.74. At [K+](o) greater than 5 mM, T-K de creased to 0.53. 3. Currents were larger in response to hyperpolarizin g voltage pulses than depolarizing pulses, indicating an inwardly rect ifying conductance. Currents were time independent except in response to voltage pulses to potentials positive to 0 mV, where the outward cu rrent decayed with an exponential time course. 4. Both the inwardly re ctifying current and the transient outward current were eliminated by the addition of 0.5 mM Ba2+, 5 mM Cs+ or 2 mM Rb+ to the extracellular solution. The current blocked by these ions reversed near the K+ equi librium potential (E(K)) over a wide range of [K+](o), indicating a hi ghly selective K+ channel. 5. The current-voltage relationship of the isolated K+ current exhibited mild inward rectification at voltages ne gative to -20 mV and a negative slope conductance at voltages positive to -20 mV. 6. The Cs+- and Ba2+-induced blocks of the K+ current were concentration dependent but voltage independent. The apparent dissoci ation constants were 0.8 mM for Cs+ and 40 mu M for Ba2+. The Kf condu ctance decreased when extracellular Na+ was removed. 7. Increasing [K](o) decreased the K-+ chord conductance (g(K)) at negative membrane p otentials. In the physiological voltage range, increasing [K+], from 2 to 5 mM caused g(K) to decrease by approximately 25 %. 8. We conclude that the inwardly rectifying K+ conductance represents the resting K conductance of the toad RPE apical membrane. The unusual properties o f this conductance may enhance the ability of the RPE to buffer [K+], changes that take place in the subretinal space at the transition betw een dark and light.