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Journal of Neuroscience, Vol 7, 2423-2432, Copyright © 1987 by Society for Neuroscience
Distribution of potassium conductance in mammalian Muller (glial) cells: a comparative study
EA Newman
The distribution of K+ conductance across the surface of retinal Muller
cells was determined in 5 mammalian species--rabbit, guinea pig, mouse, owl
monkey, and cat--and in tiger salamander. Potassium conductance was
measured by monitoring cell depolarizations evoked by focal ejections of a
high-K+ solution onto the surface of freshly dissociated cells. This
technique measured the total K+ conductance of a given cell region
(regional conductance), i.e., the specific K+ conductance times the total
surface area in that region. In mammalian species with avascular retinas
(rabbit, guinea pig), the regional K+ conductance within the middle portion
of the cell was only a fraction (10.6-28.9%) of the endfoot conductance,
while the conductance of the distal (photo- receptor) end of the cell was
approximately half (41.2-49.8%) the endfoot conductance. In 2 species with
vascularized retinas (mouse and owl monkey), by contrast, the regional K+
conductance within the middle portion of the cell was as large as
125.5-129.8% of the endfoot conductance. In these cells the K+ conductance
of the distal end was 68.3-82.9% of the endfoot value. In cat, a third
vascularized species, the K+ conductance was highest (187.1% of the endfoot
value) at the distal end of the cell. In tiger salamander, which has an
avascular retina, the regional K+ conductance of all regions distal to the
endfoot was only 2.4-15.7% of the endfoot value. Differences in the
distributions of regional K+ conductance observed in the 6 species raise
the possibility that in vascularized mammalian retinas, the high- K+
conductance of the middle portion of Muller cells is associated with
retinal blood vessels. The results are consistent with the hypothesis that,
in avascular species, Muller cells aid in regulating extracellular K+
levels by transferring (siphoning) excess K+ principally into the vitreous
humor, while in at least some vascularized species (mouse, monkey), excess
K+ is transferred by Muller cells into retinal capillaries, as well as into
the vitreous.
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