Action potential-like responses in B104 cells with low Na+ channel densities

Action potential generation and Na+ currents were studied in B104 neuroblastoma cells in vitro using the whole-cell patch-clamp method in voltage-clamp and current-clamp mode. Action potential-like responses were elicited in 38 of 42 cells, with a threshold close to -55 mV for depolarizing stimuli, and -56 mV for anode-break stimuli. Response amplitudes were larger when cells were held at more negative prepulse potentials, and were well fit by a Boltzmann distribution with a midpoint of approx. -75 mV, close to the V1/2 for Na+ current steady-state inactivation in these cells. Cells displaying action potential-like responses exhibited a peak Na+ current density of 133 +/- 0.14 pA/pF (range, 10.2-296.2 pA/pF) and a low gK:gNa ratio (0.0067 +/- 0.0023). Exposure to 0.1 mM Cd2+ did not block the generation of action potential-like responses in B104 cells, while 1 microM TTX abolished the responses. We conclude that low densities of Na+ channels (< 3/microns2, and < 1/micron2 in some cells) can support the generation of action potential-like responses in B104 cells if they are held at hyperpolarized levels to remove inactivation. The low leak and K+ conductance of these cells may contribute to their ability to generate action potential-like responses under these circumstances.