1 mV, Fig. 8) Our analysis of MK801-induced inhibition of Kv-channel currents suggests that the drug is unlikely to interact

preferentially with open or inactivated states of the Kv channels because of the following reasons. First, the inhibition was voltage-independent (Fig. 3). Many open-channel blockers inhibit voltage-gated channels in voltage-dependent manner, especially in the activation voltage range of the channels (47) and (48), because the drug-channel interaction requires channel opening and the drug-binding site is located in the Anti-cancer Compound Library transmembrane pore region. Second, the steady-state activation and inactivation of Kv channels were unaffected by MK801 treatment (Fig. 5). Although alterations in the steady-state activation and inactivation curves are not strictly required in state-dependent drug-channel interaction, most state-dependent channel blockers alter the steady-state channel kinetics (such as a left-shift of inactivation) (49) and (50). Third, when spontaneous channel activation and inactivation were prevented by holding Em at a hyperpolarized potential (−110 mV), the first depolarizing pulse after the ∼2-min treatment with MK801 produced an identical INCB018424 in vitro degree and pattern of Kv-channel inhibition as in the steady-state experiments (Fig. 4). This verifies

the hypothesis that MK801 binds Kv channels in their resting closed states and inhibits them (tonic inhibition). Fourth, the use-dependency observed in this study was minimal (Fig. 3). Although use-dependent inhibition is typically strong evidence of state-dependent inhibition, the minimal use-dependency detected here does not support the state-dependent block theory. The slow inactivation time course was markedly accelerated in the presence

of MK801 (Fig. 2). However, this does not appear to contribute out substantially to MK801 inhibition of Kv channels because of the following observation: the blockade reached maximal levels within 50 ms after application of the voltage step depolarization, when slow inactivation is apparently absent (Fig. 2 and Fig. 3A), which indicates that MK801 diminished the “peak” amplitude of the Kv-channel currents at the beginning of the depolarizing pulse. Based on these results, we suggest that MK801 inhibits Kv channels primarily by binding to the channels in their closed states and reducing channel availability or decreasing channel conductance. The blockade of Kv channels by MK801 in RMASMCs reported here is highly similar to the inhibition of the channels by ketamine (14). The ketamine block of Kv channels was also voltage-independent and did not alter steady-state channel kinetics. However, MK801 inhibits Kv channels in RMASMCs more potently (IC50 of ∼100 μM) than ketamine (IC50 of ∼500 μM).