Potassium buffering by Müller cells isolated from the center and periphery of the frog retina

Abstract

Müller (radial glial) cells span the retina from the outer to the inner limiting membranes. They are the only glial cells found in the amphibian retina. The thickness of the frog (Rana pipiens) retina decreases by a factor of about four from the center to the periphery. Thus, Müller cells were isolated, by enzymatic dissociation, with stalk lengths from 20 to 140 microm. Their ability to transfer K(+) via the stalk between soma and endfoot was studied. Membrane currents were recorded using the whole-cell voltage-clamp technique with the pipette sealed to either the endfoot or the soma. Inward (I(KIN)) or outward (I(KO)) currents were elicited by rapid increases (3 to 10 mM) or decreases (3 to 1 mM) of the extracellular K(+) concentration ([K(+)](o)) either by local application (close or distant to the recording pipette) or around the entire cell (whole cell perfusion). For the long central cells, the ratio I(KIN)/I(KO) was 4.6 +/- 0.6 SE (n = 9) at the endfoot and 1.7 +/- 0.1 SE (n = 8) at the soma. In cells from the retinal periphery, the ratio I(KIN)/I(KO) was higher, 7.0 +/- 0.27 (n = 8) at the endfoot and 3.2 +/- 0.1 (n = 10) at the soma. The results suggest that there is less inward rectification in the somatic than in the endfoot membrane. As expected from previous studies, the sensitivity of the cells to K(+) was higher at the endfoot than at the soma. The amplitude of I(KIN) at the endfoot compared to the soma was about 8-fold for the long central cells but only about 1.5-fold for the short peripheral cells. Currents spread readily from endfoot to soma in the peripheral cells. In the long central Müller cells the soma and endfoot appeared electrotonically isolated. The "functional length constant", lambda, of cell stalk processes was about 70 microm. The relative decrement of large inward currents was stronger than that of smaller outward currents; this difference ("artificial rectification") is explained by a simple model, where larger currents (inward) are attenuated more than smaller (outward) currents. The data support the hypothesis that in the retinal periphery, Müller cells provide extensive spatial K(+) buffering from both plexiform layers into the vitreous body. In the central retina, however, such currents are limited within a short (interlaminar) range.

Potassium channel activity and glutamate uptake are impaired in astrocytes of seizure‐susceptible DBA/2 mice

“…We next assessed the ability of cultured astrocytes to buffer potassium by measuring membrane currents in response to extracellular [K + ] steps (Skatchkov et al., 1999; Zhou & Kimelberg, 2000). The astrocytes were used for studies 3 weeks after plating.…”

“…The membrane potentials were determined immediately after attainment of whole‐cell mode, and cells were then subsequently held under voltage‐clamp at this potential. Applications of solutions with increased K + were used to assess potassium uptake ability of the cell as the shift in inward current following shift in [K + ] o from 3–10 or 30 m m (Skatchkov et al., 1999).…”

Molecular mechanisms of K+ clearance and extracellular space shrinkage—Glia cells as the stars

“…The ion movement needed to depolarize a cell membrane, although electrically significant, does not change ion concentrations much (Blaustein, Kao, & Matteson, ), and will therefore, in itself, not contribute quantitatively to K + clearance from the extracellular space. The driving force for K + through glial K + channels is generally outwardly directed, unless the cells are experimentally voltage‐clamped to a membrane potential more negative than the V eq(K) (Djukic, Casper, Philpot, Chin, & McCarthy, ; Neusch et al, ; Skatchkov, Krusek, Reichenbach, & Orkand, ). However, with the isopotentiality of the glial network, one may obtain channel‐mediated influx of K + under conditions where only a fraction of the electrically coupled syncytium is exposed to elevated [K + ] o , and the combined astrocytic network membrane potential remains virtually undisturbed.…”

Spatial Buffering Mechanism: Mathematical Model and Computer Simulations

“…In reality, however, the electrical properties of a glial cell membrane are not uniformly distributed. As described above, a specific example is the increased conductance at the endfeet of the Müller cells in the retina of amphibians [5,20,21,27]. Obviously, it will be interesting to find out whether the increased endfoot conductance enhances or retards the diffusion of ions via the spatial buffering mechanism.…”

“…It should be pointed out that the distribution of the Kir channels in certain glial cells, such as the Müller cells in the retina of amphibians, has been shown to be nonuniform with a preponderance at the endfoot of the vitreous humor [5,20,21,27]. This nonuniformity may manifest itself in a higher K + conductance at the endfoot processes.…”