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. We have suggested that the retinal Müller cell, a specialized astrocyte that spans nearly the entire width of the retina, buffers changes in retinal [K + ] o (4,5). We have shown that amphibian Müller cells are almost exclusively permeable to K + (6) and that 94 percent of the total K + conductance in these cells occurs in the Müller cell endfoot, a process lying adjacent to the vitreous humor (4). This highly asymmetric K + conductance distribution may make the process of K + spatial buffering more powerful than has been recognized. For example, nearly all of the K + current entering Müller cells from regions of increased [K + ] o within the retina may leave the Müller cell endfoot process at the vitreo-retinal border. Thus, the vitreous would function as a large potassium sink.We now present experimental evidence of extracellular K + buffering by Müller cells which utilizes this asymmetric conductance distribution. Dissociated Müller cells from the salamander Ambystoma tigrinum were prepared and maintained as described (4). The distal end of the Müller cell surface was exposed to increased [K + ] o by pressure-ejecting an 85 mM KCl-Ringer solution from an extracellular pipette (approximately 3 µm in tip diameter). Perfusate near this ejection pipette was drawn into a suction pipette (30 µm in diameter) to

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Measurements of absolute transretinal resistance and of the relative resistance of the various retinal layers were obtained in the frog. The resistance of clamped sections of isolated retina was 66 omega . cm2, which results in an average resistivity (rho) between inner and outer limiting membranes of 5,050 omega . cm. In eyecups, relative resistances (obtained by passing constant currents across the retina) were assigned to specific layers of the retina with the aid of physiological criteria (e.g., depths of light-evoked field potentials, changes in extracellular K+ concentration, base-line noise level, and resistance). These relative resistances were then converted to absolute values, a calculation feasible because the region between inner and outer limiting membranes, which has the same structure in both isolated and eyecup retinas, could be specified during experiments. Resistivities (in omega . cm) for the retinal layers include 1) subretinal space, 970; 2) inner and outer nuclear layers, 6,800; and 3) inner plexiform layer, 1,750. The ganglion cell and optic nerve fiber layers were too thin to resolve individually, but rho of the two layers combined was 7,900. The outer plexiform layer was also too thin to reliably resolve, but its rho is likely the same as the inner plexiform layer. The extracellular space volume fraction (alpha) of the retinal layers was estimated from these rho s, and the following values were obtained: 1) subretinal space, 0.12; 2) outer and inner nuclear layers, 0.03; 3) inner and outer plexiform layers, 0.11; and 4) ganglion cell and optic nerve fiber layers, 0.02. The decreased rho and increased alpha of the inner plexiform layer and the subretinal space, compared with that of the nuclear layers, are expected from their anatomy. A consideration of these inhomogeneities is required in analyses of field potentials and of changes in extracellular ionic concentrations.

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Dissociated cells of transporting epithelia, when cultured on an impermeant substratum, form polarized monolayers frequently characterized by the presence of domes. If the assumption is made that the monolayer exhibits a uniform stretch modulus of elasticity and tension of cell-dish adhesion, Ta, then biophysical properties of the epithelium can be predicted. We have shown that for such epithelia, domes should (a) have circular bases, (b) be sections of spheres with a constant height to radius, h/r, ratio, (c) have a dome-wall tension, Tw, that is constant, and (d) have a dome volume that is a function of radius alone. Additionally, a Laplace equation derived for this geometry predicted the hydrostatic pressure from within to outside domes as a decreasing function of radius alone. By microscopy, domes had predominantly circular bases and were found to be sections of spheres with a constant height, h, to radius, r, ratio of 0.684. Using the Laplace equation derived for this geometry and measurements of delta P and r, the tension of cell-dish adhesion, Ta, and dome-wall tension, Tw, were found to be constants of 6.60 and 7.08 torr, respectively. Combining the constants for Ta and h/r ratio, and the fact that domes are sections of spheres, delta P and dome volume were shown to be known functions of radius alone. In addition, the modulus of elasticity of the epithelium was calculated to be 4.82 X 10(3) dyn/cm2.