1. Retinal pigment epithelial (RPE) cells were isolated enzymatically from bullfrog retinae. The patch‐clamp technique was employed to investigate whole‐cell currents under voltage‐clamp conditions. 2. Isolated RPE cells were columnar or cuboidal in form, often with long processes protruding from the apical surface. Distinct apical and basal membrane domains were maintained for several hours following isolation. 3. The mean membrane capacitance was 62 pF. The resting potential averaged ‐30 mV, but it was as high as ‐75 mV in some cells. 4. Three voltage‐dependent currents were observed: a time‐independent and inwardly rectifying current and two time‐dependent outwardly rectifying currents that had distinct kinetic properties. 5. Voltage pulses from a holding potential of ‐70 mV to potentials ranging from ‐30 to ‐120 mV produced membrane currents that were essentially time independent. The I‐V relationship in this voltage range depended on the resting potential. It was usually inwardly rectifying in cells with resting potentials negative to about ‐50 mV, but tended to be linear in cells with more positive potentials. Three observations strongly suggested that the inwardly rectifying current is carried by K+. First, increasing the extracellular K+ concentration [( K+]) from 2 to 112 mM shifted the zero‐current potential of the I‐V relationship in the positive direction from an average value of ‐60 mV to 0 mV. Second, the addition of the K+ channel blockers Ba2+ (2 mM) or Cs+ (5 mM) to the extracellular solution inhibited a major component of the inwardly rectifying current. Finally, the reversal potential (Vr) of the Ba2(+)‐sensitive current averaged ‐90 mV, near the K+ equilibrium potential (EK). 6. In approximately 50% of the cells, depolarizing voltage pulses to potentials more negative than ‐30 mV evoked an outward current that resembled the delayed rectifier present in other non‐excitable cells. It activated with sigmoidal kinetics in less than 100 ms following a brief delay and then declined exponentially with a time constant of approximately 1 s. The peak chord conductance associated with this current was half‐maximal at +14 mV. Several observations indicated that this outwardly rectifying current is carried primarily by K+: its Vr closely matched EK over a wide range of extracellular [K+]; it was inhibited 80% by exposure to the K+ channel blockers 4‐aminopyridine (1 mM) and tetraethylammonium (20 mM); and it was abolished by intracellular dialysis with a K(+)‐free solution.(ABSTRACT TRUNCATED AT 400 WORDS) © 1990 The Physiological Society
