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Journal of Neuroscience, Vol 5, 2225-2239, Copyright © 1985 by Society for Neuroscience
Membrane physiology of retinal glial (Muller) cells
EA Newman
Electrophysiological techniques were used to determine the ion selectivity
properties and the spatial distribution of the membrane conductance of
amphibian Muller cells. Membrane potential changes recorded during ion
substitution experiments in frog (Rana pipiens) retinal slices demonstrated
that the Muller cell K+:Na+ membrane permeability ratio is approximately
490:1 and that cell Cl- permeability is extremely low. In frog retinal
slices, Muller cell input resistance was 8.5 megohms when measured in the
inner plexiform layer and 4.8 megohms when measured in the optic fiber
layer. Intact, enzymatically dissociated salamander (Ambystoma tigrinum)
cells had an input resistance of 7.9 megohms, whereas cells lacking their
endfoot process (removed by surgical microdissection or by shearing force)
had a resistance of 152 megohms. Pressure ejection of a 100 mM K+ solution
near the proximal surface of the endfeet of dissociated salamander cells
produced depolarizations 7 times greater than did ejections near the
lateral face of the endfoot and 24 to 50 times greater than did ejections
near other cell regions. Similar K+ ejection results were obtained from
Muller cells in salamander and frog retinal slices. Taken together, these
results demonstrate that in both the frog and the salamander, approximately
95% of the total membrane conductance of Muller cells is localized in the
cell's endfoot process. In salamander, the specific membrane resistance of
the endfoot membrane was estimated to be 32 ohm X cm2 whereas the specific
resistance of the remainder of the cell was 7300 ohm X cm2. This remarkably
nonuniform conductance distribution has important consequences for theories
concerning K+ regulation in the retina and for mechanisms underlying
electroretinogram generation.
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