ACTION POTENTIAL-LIKE RESPONSES IN B104 CELLS WITH LOW NA+ CHANNEL DENSITIES

Action potential generation and Na+ currents were studied in B104 neur oblastoma cells in vitro using the whole-cell patch-clamp method in vo ltage-clamp and current-clamp mode. Action potential-like responses we re elicited in 38 of 42 cells, with a threshold close to -55 mV for de polarizing stimuli, and -56 mV for anode-break stimuli. Response ampli tudes were larger when cells were held at more negative prepulse poten tials, and were well fit by a Boltzmann distribution with a midpoint o f approx. -75 mV, close to the V-1/2,,, for Na+ current steady-state i nactivation in these cells. Cells displaying action potential-like res ponses exhibited a peak Na+ current density of 133 +/- 0.14 pA/pF (ran ge, 10.2-296.2 pA/pF) and a low g(K):g(Na) ratio (0.0067 +/- 0.0023). Exposure to 0.1 mM Cd2+ did not block the generation of action potenti al-like responses in B104 cells, while 1 mu M TTX) abolished the respo nses. We conclude that low densities of Na+ channels (<3/mu m(2), and <1/mu m(2) in some cells) can support the generation of action potenti al-like responses in B104 cells if they are held at hyperpolarized lev els to remove inactivation. The low leak and K+ conductance of these c ells may contribute to their ability to generate action potential-like responses under these circumstances.