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_V1.1 and K_V4.1 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.