Ion-selective double-barreled microelectrodes (ISME) were used to measure intracellular K+ (aKi), Na+ (aNai), and Cl- (aCli) activities of neuropile glial (NG) cells in the central nervous system of the medicinal leech Hirudo medicinalis. Ion fluxes were induced by an increase in extracellular K+ concentration [( K+]o) and analyzed to elucidate the ionic mechanism of the K+ uptake occurring under such conditions. 2. In addition, the K+ concentration of the extracellular space of the nerve cell body region (NCBR) and the neuropile (N) was measured with neutral carrier K(+)-ISME. In normal saline (4 mM K+), a concentration of 4.2 mM was measured in both extracellular spaces. No differences between the K+ concentration of the bathing fluid and the extracellular spaces were found at higher (i.e., 10 and 40 mM) K+ concentrations. 3. In normal saline, the mean membrane potential (Em) was -68 mV, and the mean aKi, aNai, and aCli were found to be 77, 10, and 7 mM, respectively. The corresponding equilibrium potentials were -81, 56, and -66 mV. The chloride equilibrium potential (ECl) was similar to Em, and it is concluded that chloride is passively distributed across the NG cell membrane. 4. When [K+]o was transiently increased 10-fold (i.e., to 40 mM), aKi and a Cli increased transiently by 22 and 25 mM, respectively, and the membrane depolarized to -28 mV, which was similar to both K+ equilibrium potential (EK) and ECl. The KCl uptake was accompanied by a transient decrease in aNai to 5 mM. 5. After incubation for at least 1 h in Na(+)-free saline, NG cells accumulated K+ in the absence of extracellular Na+ to levels similar to those observed in the presence of Na+. Therefore the uptake of K+ was not dependent on external--and probably also internal--Na+. 6. Changes in cell volume induced by the increase in [K+]o were estimated by loading NG cells with choline and monitoring its intracellular concentration with Corning-K(+)-ISME. In saline containing 40 mM K+, NG cell volume increased to approximately 150% of its volume in normal saline. 7. It is concluded that the mechanism of K+ uptake in NG cells is by passive KCl and water influx, which causes cell swelling.
