Dm. Linn et al., THE ROLE OF POTASSIUM CONDUCTANCE IN THE GENERATION OF LIGHT RESPONSES IN MULLER CELLS OF THE TURTLE RETINA, Visual neuroscience, 15(3), 1998, pp. 449-458
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.