## Detailed characterization of the voltage-gated Na^{+} and K^{+} currents.

(**A**) Na^{+} currents evoked by depolarizing pulses from −70 to +60 mV in 10 mV increments in a 2-day-old patch-clamped differentiating chondrocyte. (**B**) The current–voltage relationship of the Na^{+} current determined from the peak current at each voltage. (**C**) The functions describing the voltage dependence of steady-state activation (G–V curve) and inactivation mark a potential window for Na^{+} channel operation. The G–V curve was constructed from the current–voltage relationship. The voltage dependence of steady-state inactivation was obtained by holding the cells at the indicated holding potentials for 15 s then the peak current was recorded during a pulse to 0 mV. Data points were fitted with Boltzmann-functions yielding the half-activation (V_{1/2,a} = −36.8 mV) and half-inactivation (V_{1/2,i} = −72.4 mV) voltages. (**D**) Dose–response function of tetrodotoxin on Na^{+} channels of differentiating chondrocytes. Fitting the Hill equation to the data points yielded K_{d} = 12 nM and n_{H} = 0.87. (**E**) Voltage dependence of the steady-state activation (G–V curve) of voltage-gated K^{+} channels in chondrocytes. The G–V relationship was constructed from the current–voltage relationship at each test potential using E_{rev} = −85 mV; points were fitted with a Boltzmann-function. The curve represents a mixture of K_{V}1.1 and K_{V}4.1 channels, and the obtained V_{1/2} = −15.5 mV lies between the V_{1/2} of those channels. (**F**) Normalized current traces recorded from three different cells representing the highly variable inactivation rate of the K^{+} current. Cells were depolarized to +50 mV for 1.5 s, the first 1 s is shown for clarity.