A TETRAETHYLAMMONIUM-INSENSITIVE INWARD RECTIFIER K-SCRIPTA ELEGANS) RETINA( CHANNEL IN MULLER CELLS OF THE TURTLE (PSEUDEMYS)

Ion channels present in isolated glial (Muller) cells from the retina of the turtle (Pseudemys scripta elegans) were studied with the patch clamp technique. The predominant conductance in these cells was due to an inward rectifying potassium current. The whole-cell conductance of the inward rectifier was 20.2 +/- 1.9 nS (n = 7 cells) in a standard extracellular saline solution (3 mM extracellular potassium). This con ductance was dependent on the extracellular potassium concentration, w ith a 2.88-fold change in conductance per tenfold shift in concentrati on. The relative permeability sequence to potassium of the inward rect ifier was found to be: potassium (1.0) > rubidium (0.7) > ammonium (0. 2) > lithium (0.1) = sodium (0.1), which corresponded to the Eisenman sequence IV or V for a strong-field-strength potassium binding site on the channel. The single channel conductance measured in cell-attached patches with potassium chloride (150 mM) in the pipette was 68.5 +/- 6.0 pS (n = 3 patches). The inward rectifier current was not blocked b y extracellular tetraethylammonium (TEA(+), 20 mM), but was blocked by extracellular barium (5 mM) or cesium (5 mM). The TEA(+) insensitivit y of the inward rectifier potassium channel in Muller cells is unusual , given that this type of channel in most excitable cells is sensitive to micromolar concentrations of this compound, and may be a character istic of inward rectifier potassium channels that are primarily involv ed with extracellular potassium regulation.