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. 1984 Sep 14;225(4667):1174-5.
doi: 10.1126/science.6474173.

Control of extracellular potassium levels by retinal glial cell K+ siphoning

E A Newman, D A Frambach, L L Odette

Control of extracellular potassium levels by retinal glial cell K+ siphoning

E A Newman et al. Science. .

Abstract

Efflux of K+ from dissociated salamander Müller cells was measured with ion-selective microelectrodes. When the distal end of an isolated cell was exposed to high concentrations of extracellular K+, efflux occurred primarily from the endfoot, a cell process previously shown to contain most of the K+ conductance of the cell membrane. Computer simulations of K+ dynamics in the retina indicate that shunting ions through the Müller cell endfoot process is more effective in clearing local increases in extracellular K+ from the retina than is diffusion through extracellular space.

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Figures

Fig. 1
Fig. 1
Measurement of K+ efflux from a dissociated Ambystoma tigrinum Müller cell. (A) Schematic of a dissociated cell showing location of K+ ejection and suction pipettes and the four positions of the ion-selective microelectrode (a through d). (B) Voltage records from the ion-selective microelectrode made at the locations indicated in (A). The onset and duration of a 50-msec pressure pulse applied to the ejection pipette is indicated at the bottom. Traces b, c, and d are expanded vertically relative to trace a. The concentration scales (mM) were determined by calibrating ion-selective pipettes in a series of K+ solutions.
Fig. 2
Fig. 2
Computer simulation of the clearance of extracellular K+ from the inner plexiform layer of the frog retina after an imposed increase in [K+]o. The time course of K+ clearance achieved by diffusion through extracellular space (Diffusion), by K+ current through Müller cells (Müller cell), and by both mechanisms (Both) are shown. [K+]o was calculated at 30 percent retinal depth, the midpoint of the imposed increase. The following parameters differed from those used in the version of the model previously described (5). Active uptake was assumed not to occur. Müller cell conductance [see (12)] at the end-foot layer (0 to 4 percent retinal depth) was 32 S/cm3 and in the remainder of the cell (5 to 70 percent depth) was 0.62 S/cm3. Location (in percent retinal depth) and extracellular volume fraction (in percent, indicated in parentheses) of retinal layers: endfoot, 0 to 4 (100); optic fiber, 5 to 10 (2.4); ganglion cell, 11 to 19 (2.4); inner plexiform, 20 to 40 (13.3); inner nuclear, 41 to 57 (3); outer plexiform, 58 to 61 (13.8); and outer nuclear, 62 to 70 (3 percent). Volume fraction values from (13).
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