Voltage-dependent sodium channels in human small-cell lung cancer cells: Role in action potentials and inhibition by lambert-eaton syndrome IgG

Abstract

Sodium channels of human small-cell lung cancer (SCLC) cells were examined with whole-cell and single-channel patch clamp methods. In the tumor cells from SCLC cell line NCI-H146, the majority of the voltage-gated Na⁺ channels are only weakly tetrodotoxin (TTX)-sensitive (K _d =215 mm). With the membrane potential maintained at −60 to −80 mV, these cells produced all-or-nothing action potentials in response to depolarizing current injection (>20 pA). Similar all-ornothing spikes were also observed with anodal break excitation. Removal of external Ca²⁺ did not affect the action potential production, whereas 5 μm TTX or substitution of Na⁺ with choline abolished it. Action potentials elicited in the Ca²⁺-free condition were reversibly blocked by 4 mm MnCl₂ due to the Mn²⁺-induced inhibition of voltage-dependent sodium currents (I _Na). Therefore, Na⁺ channels, not Ca²⁺ channels, underlie the excitability of SCLC cells. Whole-cell I _Na was maximal with step-depolarizing stimulations to 0 mV, and reversed at +45.2 mV, in accord with the predicted Nernst equilibrium potential for a Na⁺-selective channel. I _Na evoked by depolarizing test potentials (−60 to +40 mV) exhibited a transient time course and activation/ inactivation kinetics typical of neuronal excitable membranes; the plot of the Hodgkin-Huxley parameters, m_∞ and h_∞, also revealed biophysical similarity between SCLC and neuronal Na⁺ channels. The single channel current amplitude, as measured with the inside-out patch configuration, was 1.0 pA at −20 mV with a slope conductance of 12.1 pS. The autoantibodies implicated in the Lambert-Eaton myasthenic syndrome (LES), which are known to inhibit I _Ca and I _Na in bovine adrenal chromaffin cells, also significantly inhibited I _Na in SCLC cells. These results indicate that (i) action potentials in human SCLC cells result from the regenerative increase in voltage-gated Na⁺ channel conductance; (ii) fundamental characteristics of SCLC Na⁺ channels are the same as the classical sodium channels found in a variety of excitable cells; and (iii) in some LES patients, SCLC Na⁺ channels are an additional target of the pathological IgG present in the patients' sera.