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 exam ined with whole-cell and single-channel patch clamp methods. In the tu mor cells from SCLC cell line NCI-H146, the majority of the voltage-ga ted Na+ channels are only weakly tetrodotoxin (TTX)-sensitive (K-d = 2 15 nm). With the membrane potential maintained at -60 to -80 mV, these cells produced all-or-nothing action potentials in response to depola rizing current injection (>20 pA). Similar all-or-nothing spikes were also observed with anodal break excitation. Removal of external Ca2+ d id not affect the action potential production, whereas 5 mu M TTX or s ubstitution of Na+ with choline abolished it. Action potentials elicit ed in the Ca2+-free condition were reversibly blocked by 4 mM MnCl2 du e to the Mn2+-induced inhibition of voltage-dependent sodium currents (I-Na). Therefore, Na+ channels, not Ca2+ channels, underlie the excit ability of SCLC cells. Whole-cell I-Na was maximal with step-depolariz ing 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 o f neuronal excitable membranes; the plot of the Hodgkin-Huxley paramet ers, m(infinity) and h(infinity), also revealed biophysical similarity between SCLC and neuronal Na+ channels. The single channel current am plitude, 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 i mplicated in the Lambert-Eaton myasthenic syndrome (LES), which are kn own to inhibit I-Ca and I-Na in bovine adrenal chromaffin cells, also significantly inhibited I-Na in SCLC cells. These results indicate tha t (i) action potentials in human SCLC cells result from the regenerati ve increase in voltage-gated Na+ channel conductance; (ii) fundamental characteristics of SCLC Na+ channels are the same as the classical so dium channels found in a variety of excitable cells; and (iii) in some LES patients, SCLC Na+ channels are an additional target of the patho logical IgG present in the patients' sera.