Intracellular chloride modulates A-type potassium currents in astrocytes

Application of the GABA(A) receptor agonist muscimol to astrocytes in situ or in vitro results in a receptor-mediated Cl- current with a concomitant block of outward K+ currents. The effect on K+ current is largely selective for the inactivating A-type current. Parallel experiments with various Cl- pipette concentrations show a significant reduction in A-type current under low Cl- conditions with minimal effect on delayed current. In addition, lower Cl- conditions caused a depolarizing shift of steady-state inactivation (V-1/2, -68 to -57 mV) and activation (V-1/2, -5.S to 34 mV) kinetics of A-type current only. Cl- had no effect on the time course of inactivation or reactivation kinetics, suggesting the Cl--mediated effect is largely on activation kinetics, indirectly affecting steady-state inactivation. Muscimol application to astrocytes under perforated patch control (gramicidin) displayed a similar block of A-type current to that of conventional whole cell patch at 40 or 20 mM pipette Cl- concentrations. With barium application under perforated patch conditions, the study of muscimol-mediated Cl- current in isolation of the effect on K+ currents was possible. This allowed estimation of intracellular Cl- concentration from receptor current reversal information. The average intracellular Cl- concentration was found to be 29 +/- 3.2 mM. The effect on activation kinetics and lack of effect on time course of inactivation or reactivation suggest that intracellular anion concentrations have an effect on the K+ channel voltage sensor region. Cl- may modulate K+ currents by altering membrane field potentials surrounding K+ channel proteins.