Potassium buffering by Muller cells isolated from the center and peripheryof the frog retina

Muller (radial glial) cells span the retina from the outer to the inner lim iting membranes. They are the only glial cells found in the amphibian retin a. The thickness of the frog (Rana pipiens) retina decreases by a factor of about four from the center to the periphery. Thus, Muller cells were isola ted, by enzymatic dissociation, with stalk lengths from 20 to 140 mu m Thei r ability to transfer K+ via the stalk between soma and endfoot was studied . Membrane currents were recorded using the whole-cell voltage-clamp techni que with the pipette sealed to either the endfoot or the soma. Inward (I-KI N) 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)) e ither by local application (close or distant to the recording pipette) or a round the entire cell (whole cell perfusion). For the long central cells, t he 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/IKO 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 rectificat ion 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 sh ort peripheral cells. Currents spread readily from endfoot to soma in the p eripheral cells. In the long central Muller cells the soma and endfoot appe ared electrotonically isolated. The "functional length constant", lambda, o f cell stalk processes was about 70 pm. The relative decrement of large inw ard currents was stronger than that of smaller outward currents; this diffe rence ("artificial rectification") is explained by a simple model, where la rger currents (inward) are attenuated more than smaller (outward) currents. The data support the hypothesis that in the retinal periphery, Muller cell s provide extensive spatial K+ buffering from both plexiform layers into th e vitreous body. In the central, retina, however, such currents are limited within a short (interlaminar) range. (C) 1999 Wiley-Liss, Inc.