Detailed characterization of the voltage-gated Na<sup>+</sup> and K<sup>+</sup> currents.

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