On the sodium and potassium currents of a human neuroblastoma cell line

Abstract

1. The patch-clamp method was applied to the study of ionic currents activated by depolarization of undifferentiated IMR-32 human neuroblastoma cells. Whole-cell sodium and potassium currents and single potassium ion channel currents from cell-attached patches were investigated. 2. Cells had a mean resting potential of -38 mV and mean input resistance of 1.6 G omega. Single action potentials were evoked under current clamp during the injection of depolarizing currents. 3. A voltage-dependent inward sodium current was observed which reversed at +44 mV. A Boltzmann fit to the activation curve gave a half-maximal activation voltage of -41.6 mV and a 'slope' of 3.9 mV. The steady-state inactivation curve had a half-maximal inactivation voltage of -81 mV and a 'slope' of 9.7 mV. 4. The time-dependent activation and inactivation of the current displayed classical Hodgkin-Huxley kinetics. Values for the time constants tau m and tau h of 0.16 and 0.63 ms were calculated for a voltage jump from -80 to -10 mV; tau m and tau h decreased as the step potential was changed from -30 to +20 mV. 5. Outward currents were activated in bathing solutions substantially free of anions and could thus be attributed to potassium ions. The tail current reversed in direction on repolarization to -60 mV when the potassium concentration in the bathing solution was increased from 6 to 30 mM. When the bathing solution contained 145 mM-potassium, and the patch pipette, 95 mM, a depolarization to -10 mV from a holding potential of -60 mV evoked an inward current. 6. Outward currents were examined by using voltage pulses which depolarized the cell to -20 mV, or more positive values, from a holding potential of -80 mV and by pulses which depolarized the cell to 0 mV, or to positive values, from a holding potential of -30 mV. A Boltzmann fit of typical activation data gave a half-maximal activation voltage of 17 mV and a 'slope' of 14 mV. 7. The time course of the rising phase of the current was described by a function of the form A(1-exp[-(t-delta t)/tau]), where delta t varied between 1 and 4 ms and tau varied between 4 and 27 ms, decreasing with increasing depolarization. There was no evidence for a fast transient component. 8. The amplitude of outward currents was reduced by extracellular calcium ions, cobalt ions, tetraethylammonium and 4-aminopyridine.(ABSTRACT TRUNCATED AT 400 WORDS)