Negative Shift In Voltage Dependence Research Articles

Voltage-clamp study of calcium currents during differentiation in the NCB-20 neuronal cell line.

1. Calcium currents (ICa) were studied in voltage-clamped NCB-20 cells. In undifferentiated cells, voltage steps from hyperpolarized potentials (-80/-100 mV) essentially revealed transient ICa showing characteristics classically described for "T-type" channels. In about 50% of the cells, there was a residual current at the end of the step; no ICa was elicited from a holding potential of -50 mV. 2. In contrast, 100% of the cells differentiated with dibutyryl cyclic AMP (cAMP) displayed a residual current in addition to the transient one, and depolarizing steps from a holding potential of -50 mV induced a sustained current. In these cells, Bay K 8644 elicited both a negative shift in voltage dependence and a moderate increase of the sustained component. 3. Although these changes in Ca2+ channel physiology result from chemically induced differentiation, they might not be directly related to the concomitant morphologic differentiation. 4. In undifferentiated NCB-20 cells, T-type Ca2+ currents can be elicited in relative isolation.

Competitive blockage of the sodium channel by intracellular magnesium ions in central mammalian neurones.

The aim of this study was to determine from macroscopic current analysis how intracellular magnesium ions, Mgi2+, interfere with sodium channels of mammalian neurones. It is reported here that permeation across the sodium channel is voltage- and concentration-dependently reduced by Mgi2+. This results in a general reduction of sodium membrane conductance and an outward sodium peak current at large positive potentials. 30 mM Mgi2+ leads ot a negative shift of voltage dependence of sodium channel gating parameters, probably due to the surface potential change of the membrane. This shift alone is, however, insufficient to explain the reduction of outward sodium currents. The blockage by Mgi2+ is decreased upon increasing intracellular or extracellular Na+ concentration, which suggests that Mgi2+ interferes with sodium permeation by competitively occupying sodium channels. Using a kinetic model to describe the sodium permeation, the dissociation constant (at zero membrane potential) of Mgi2+ for the sodium channel has been calculated to be 8.65 +/- 1.51 mM, with its binding site located at 0.26 +/- 0.05 electrical distance from the inner membrane. This dissociation constant is smaller than that of Nai+, which is 83.76 +/- 7.60 mM with its binding site located at 0.75 +/- 0.23. The low dissociation constant of Mgi2+ reflects its high affinity for the sodium channel.