Interaction of riluzole with the closed inactivated state of Kv4.3 channels.

The effect of riluzole on Kv4.3 was examined using the whole-cell patch-clamp technique. Riluzole inhibited the peak amplitude of Kv4.3 in a reversible, concentration-dependent manner with an IC(50) of 115.6 microM. Under control conditions, a good fit for the inactivation of Kv4.3 currents to a double exponential function, with the time constants of the fast component (tau(f)) and the slow component (tau(s)), was obtained. tau(f) was not altered by riluzole at concentrations up to 100 microM, but tau(s) became slower with increasing riluzole concentration, resulting in the crossover of the currents. The inhibition increased steeply with increasing channel activation at more positive potentials. In the full activation voltage range positive to (+)30 mV, however, no voltage-dependent inhibition was found. Riluzole shifted the voltage dependence of the steady-state inactivation of Kv4.3 in the hyperpolarizing direction in a concentration-dependent manner. However, the slope factor was not affected by riluzole. The K(i) for riluzole for interacting with the inactivated state of Kv4.3 was estimated from the concentration-dependent shift in the steady-state inactivation curve and was determined to be 1.2 muM. Under control conditions, closed state inactivation was fitted to a single exponential function. Riluzole caused a substantial acceleration in the closed state inactivation. In the presence of riluzole, the recovery from inactivation was slower than under control conditions. Riluzole induced a significant use-dependent inhibition of Kv4.3. These results suggest that riluzole inhibits Kv4.3 by binding to the closed inactivated state of the channels and that the unbinding of riluzole occurs from the closed state during depolarization.