THE ROLE OF POTASSIUM CONDUCTANCE IN THE GENERATION OF LIGHT RESPONSES IN MULLER CELLS OF THE TURTLE RETINA

Muller cells are highly permeable to potassium ions and play a major r ole in maintaining potassium homeostasis in the vertebrate retina duri ng light-evoked neuronal activity. Potassium fluxes across the Muller cell's membrane are believed to underlie the light-evoked responses of these cells. We studied the potassium currents of turtle Muller cells in the retinal slice and in dissociated cell preparations and their r ole in the genesis of the light-evoked responses of these cells. In ei ther preparation, the I-V curve, measured under voltage-clamp conditio ns, consisted of inward and outward currents. A mixture of cesium ions , TEA, and 4-AP blocked the inward current but had no effect on the ou tward current. Extracellular cesium ions alone blocked the inward curr ent but exerted no effect on the photoresponses. Extracellular barium ions blocked both inward and outward currents, induced substantial dep olarization, and augmented the light-evoked responses, especially the OFF component. Exposing isolated Muller cells to a high potassium conc entration did not cause any current or voltage responses when barium i ons were present. In contrast, application of glutamate in the presenc e of barium ions induced a small inward current that was associated wi th a substantially augmented depolarizing wave relative to that observ ed under control conditions. This observation suggests a role for an e lectrogenic glutamate transporter in generating the OFF component of t he turtle Muller cell photoresponse.