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

Abstract

 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⁺.