Electrophysiological measurements of volume changes in leech neuropile glial cells

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Abstract
Double-barrelled microelectrodes, sensitive to quaternary ammonium ions, were used for simultaneous measurements of the intracellular free concentrations of choline ([Ch]i) or tetramethylammonium [TMAi] as well as membrane potential (Em) in neuropile glial cells of the leech, Hirudo medicinalis. Bath application of Ch or TMA (5 mM, 1 min) resulted in a transient membrane depolarization accompanied by a long- lasting (0.5–1 h) intracellular accumulation of these compoundsto levels of between 5 and 15 mM. Changes in [Ch]i or [TMA]i were used for the calculation of changes in relative cell volume. Elevation of the extracellular K+ concentration ([K+]e) from 4 to 9,15,21,27.5, or 40 mM elicited a membrane depolarization and a reversible cell swelling by about 7.5,14, 18.5, 27 and 50%, whereas reduction of [K+]e to 1.5 mM as well as bath application of serotonin (5-HT) produced a membrane hyperpolarization and a concomitant shrinkage by about 6 and 14.3%, respectively. The measured alterations in cell volume were compared with calculated data based on the assumption of an osmotic equilibrium disturbed by potential-dependent changes of the intracellular Cl− concentration. The results indicate, that K+- and serotonin-induced changes in the cell volume of the neuropile glial cells are due to passive KCl and water fluxes.