CHARACTERIZATION OF AN INWARD-RECTIFYING POTASSIUM CURRENT IN NG108-15 NEUROBLASTOMA X GLIOMA-CELLS

By using the whole-cell patch-clamp technique, an inwardly rectifying potassium current, which resembled the ''classic'' inward-rectifying p otassium current (I-KIR) of other cells in terms of electrophysiologic al and pharmacological properties, was identified in db-cAMP-different iated NG108-15 cells. First, the current was dependent on voltage and time. It could be elicited by applying an initial depolarizing prepuls e and a subsequent hyperpolarizing command pulse to the cell. The ampl itude of the current depended on both the prepulse and the command pul se and increased with the hyperpolarization of the command pulse as we ll as the depolarization and the prolongation of the prepulse. The act ivation and inactivation of the current could be fitted well by single -exponential functions and increased with the hyperpolarization of the membrane. Second, the current was dependent on the extracellular pota ssium concentration ([K+](o)). Elevation of [K+](o) resulted in a mark ed increase in the cut-rent amplitude and a positive shift of the peak -current/voltage curve as well as the reversal potential. A tenfold in crease of [K+](o) introduced an approximate to 43-mV shift of the reve rsal potential, indicating that the current was carried mainly by K+. The conductance (g/g(Max)) of the current was also dependent on the [K +](o) and increased with increases in [K+](o) in a manner approximatel y proportional to the square-root of [K+](o). Finally, the current was sensitive to Cs+ (1 mmol/l), Ba2+ (1 mmol/l) and quinidine (0.2 mmol/ l); whereas, two typical potassium channel inhibitors, tetraethylammon ium (TEA) and 4-aminopyridine (4-AP), were weak blockers and reduced t he current at high concentration (>10 mmol/l). It was also observed th at the current was depressed by Cd2+ (1 mmol/l) and Co2+ (1 mmol/l) an d increased by perfusing the cell with Ca2+-free solution. Thus, excep t for the sensitivity to Cd2+, Co2+ and Ca2+, the current displayed mo st of the hallmarks described for the ''classic'' I-KIR. In conclusion , there appears to be a voltage-dependent I-KIR-type inward rectifier in db-cAMP-differentiated NG108-15 cells.