Following a report of voltage-gated Na+ and K+ currents in cultured Schwann cells (Chiu, Shrager & Ritchie, 1984), various kinds of voltage-gated ionic channels have been found in glial cells in the peripheral and central nervous systems (Barres, Chun & Corey, 1990). Among these ionic channels in glial cells, inwardly rectifying potassium (Kir) channels are important for the regulation of the potassium microenvironment in the nervous system by potassium siphoning (Newman, Frambach & Odette, 1984; Konishi, 1990) or spatial potassium buffering (Orkand, Nicholls & Kuffler, 1966). This chapter focuses on the mechanism of the expression of functional Kir channels in mouse Schwann cells in relation to axonal contact, intracellular cAMP and neuronal activity, and discusses the physiological significance of Schwann cells in potassium regulation.
In cultured Schwann cells obtained from dissociated sciatic nerves of neonatal mice, only voltage-gated outward currents were recorded with the whole-cell patch-clamp technique during depolarizing voltage steps (Konishi, 1989). The equilibrium potentials of the tail currents indicated that these outward currents were carried by potassium ions (Konishi, 1989). They were eliminated by bath application of quinine, but were not affected by external barium (Konishi, 1990). In freshly dissociated Schwann cells from neonatal sciatic nerves, both myelinating and non-myelinating cells showed bariumsensitive inward currents during hyperpolarizing voltage steps, which were disclosed by subtracting a record in a solution containing barium from a record in standard solution (Fig. 4.1) (Konishi, 1992).