Heterogeneous expression of Ca2+-dependent K+ currents by Muller glial cells

The flavoid phloretin was used to study large-conductance, Ca2+-activated K+ (BKCa) channel-mediated currents in isolated porcine Muller (retinal glial) cells. In excised membrane patches, application of phloretin (100 mu M) increased the open probability of single BKCa channels. In whole-cell records, phloretin increased the amplitude of an outward current which was blockable by iberiotoxin (40 nM) and TEA (1 mM), and, thus, was identified as a BKCa current. Only 50% of the investigated cells expressed phloretin-sensitive BKCa currents. Cells with phloretin-sensitive BKCa currents responded to glutamate (500 mu M) with an increase of the amplitude of this current, whereas glutamate had no effects on the K+ currents of cells without BKCa currents. As glutamate is a major neurotransmitter of the retina, and the membrane properties of Muller cells are largely determined by their K+ channels, the observed data may shed new light onto the functions of retinal glia.