Potassium current suppression by quinidine reveals additional calcium currents in neuroblastoma cells

Quinine and quinidine have been evaluated with regard to their effects on the electrical activity of neuroblastoma cells. Under voltage-clamp conditions, we have found that quinine and quinidine block both the voltage-dependent and Ca2+-dependent K+ conductances. Blockage of the voltage-dependent K+ channel is manifest as an increase in the amplitude and in the duration of the action potential. Blockage of the Ca2+-dependent K+ channel in Na+-free (replaced by Tris) solutions containing 6.8 mM Ca2+ and tetraethylammonium ion or 4-aminopyridine (to block the voltage-dependent K+ current) is seen as a further prolongation of the Ca2+ action potential and diminution of the after-hyperpolarization. A critical role of the Ca2+-dependent K+ conductance in modulation of the rate and duration of trains of Ca2+ action potentials is shown by the use of low concentrations (5-40 microM) of quinine or quinidine, which diminish the Ca2+-dependent K+ conductance in a graded manner. After complete blockade of K+ currents, the peak Ca2+ currents are enhanced at all voltages, especially at values more positive than -30 mV, where a steady-state inward current appears as well. In this same voltage range, the decay of the Ca2+ current exhibits two time constants--that of the transient inward current, which is about 20 msec, and a much slower (approximately 2000 msec) component. It is suggested that neuroblastoma cells have two types of calcium channels--one which generates the Ca2+ action potential and a second, distinguished by activation at more depolarized levels and by a slow rate of inactivation, which underlies the calcium entry necessary to activate the Ca2+-dependent K+ conductance.