Sodium-sensitive, calcium-independent potassium conductance in the leech giant glial cell

We have measured membrane current, membrane potential and intracellular Na+ and Ca2+ concentrations, [Na+]i and [Ca2+]i, of the giant glial cell in the nervous system of the leech Hirudo medicinalis using conventional microelectrodes and the fluorescent dyes sodium-binding benzofuran isophthalate (SBFI) and fura-2. When the Na+ was removed from the saline, the membrane conductance increased twofold from 1.29±0.1 µS to 2.57±0.18 µS (mean ± SEM; n=27). The rise in membrane conductance was accompanied by a current, which reversed around –74 mV, and the amplitude of K+-induced depolarizations or currents increased during Na+ removal, suggesting an increase in the K+ conductance of the glial membrane. We also monitored [Ca2+]i when removing external Na+ in the presence and absence of external Ca2+, and during injection of the Ca2+-chelator BAPTA into the cells. Our results indicate that Na+ modulates a K+ conductance of these glial cells, independent of intra- and extracellular Ca2+.