VOLTAGE-DEPENDENT SODIUM-CHANNELS IN HUMAN SMALL-CELL LUNG-CANCER CELLS - ROLE IN ACTION-POTENTIALS AND INHIBITION BY LAMBERT-EATON SYNDROME IGG

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 nm). 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-or-nothing spikes were also observed with anodal break excitation. Removal of external Ca2+ did not affect the action potential production, whereas 5 mu M 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(infinity) and h(infinity), 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.