A stable internal environment (Claude Bernard's concept of le maintenance de la milieu intérieur) has long been recognized as an essential requirement for cells to carry out their normal activities. Nerve cells, for example, are particularly sensitive to fluctuations in extracellular potassium ions [ K+]o because even a small rise represents a large fractional change from the baseline level of about 3 mM. The increase in [K+]o can be far greater under pathological conditions, resulting in disruption of neuronal function and eventual tissue damage (Szatkowski and Attwell, 1994).
A series of landmark studies by Stephen Kuffler and his colleagues on the electrophysiological properties of glial cells led to an appreciation of their importance in potassium homeostasis in the nervous system (Kuffler and Potter, 1964; Kuffler et al., 1966; Orkand et al., 1966). Intracellular recordings from glial cells in the amphibian optic nerve showed that the glial cell membrane potential was almost completely dependent upon the extracellular concentration of potassium ions [K+]o. As a result, resting potentials were close to -90 mV (about 10-20 mV more negative than in neurons), and over a large range of [K+]o, the concentration dependence was in accordance with the Nernst equation, i.e., a ten-fold change in K+ (at T = 24°C) resulted in a 59 mV change in membrane potential.
Subsequently, this log-linear relationship was found also with frog Müller cells (Fig 5.1, continuous line). However, in both astrocytes and Müller cells, the membrane potential shows a departure from the Nernst equation at low potassium concentrations, due to a small but significant membrane permeability to Na+ (Newman, 1985a; Conner et al., 1985; Reichenbach and Eberhardt, 1986). Thus, substituting a nonpermeant ion (e.g., choline) for sodium in the extracellular bathing solution extends the log-linear relation (Fig. 5.1, dotted line).
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