Potassium Uptake Mechanisms of Cultured Oligodendrocytes Studied with Ion-Sensitive Electrodes

Abstract

Physiological studies over the past two decades of the functional roles of glial cells in the operation of the nervous system have focused attention on the abilities of these cells to regulate ions and take up amino acids in the neuronal microenviron- ment [17]. Because glial membranes are selectively permeable to K⁺ [9,10] and the regulation of this ion is important in the control of nerve transmission [15], special attention has been paid to a possible role of glial cells in K⁺ homeostasis [5, 8,18]. Following the observation in the bee retina, made with the use of ion-sensitive electrodes, that during photoreceptor stimulation the K⁺ activity [K⁺]_i of glial cells increased [3], the question arose as to the mechanisms producing this increase. Two types of processes have been considered: (a) space-independent net uptake; and (b) space-dependent uptake via spatial buffer currents [1, 3–6, 9, 11]. With the first mechanism, K⁺ is taken up by a transport mechanism as a consequence of a rise in [K⁺]_O which may be uniform over the cell surface. For the second, an uneven distribution of K⁺ produces a current which results from the difference between the membrane potential V _m and the potassium equilibrium potential E _k . This current drives K⁺ into the glial cell in regions where [K⁺]_O is elevated and out of the cells where [K⁺]_O is low. Such a mechanism depends on a high relative K⁺ permeability. In cell culture, one has the advantage of having independent control over both the ionic environment and membrane currents of glial cells. We have used oligodendrocytes in culture to study K⁺ uptake and membrane permeability.