Ckade of hKv4.3 channels by acacetin was determined at 0.2, 1, 2, and 3.3 Hz using a train of 20 pulses of a 200-ms voltage step. Figure 5A shows the normalized hKv4.3 47931-85-1 web current traces recorded at 3.3 Hz in a representative cell before and after application of 3 mM acacetin. Though hKv4.3 current showed a significant use-dependent inhibition in control, the use-dependent blockade was evident with acacetin. The current blockade at the first pulse was less than that at following pulses. The fractional blockade of hKv4.3 current at each frequency with 3 mM acacetin is illustrated in Fig. 5B. The use-dependent blockade was enhanced as the depolarization frequency was increased. In addition, acacetin exhibited more potent of inhibition on hKv4.3 current at 3.3 Hz than that at 0.2 Hz and 1.0 Hz. Figure 5C shows the percentage current at the 20th pulse of different frequencies with 0.1?00 mM acacetin. The curves were fitted to a Hill equation to obtain the IC50. The IC50 of acacetin for inhibiting hKv4.3 current was reduced as increase of the depolarization frequency (IC50: 7.9, 6.1, 3.9, and 3.2 mM at 0.2, 1.0, 2.0, and 3.3 Hz, respectively).Effect of acacetin on closed-state inactivation of hKv4.3 currentThe 3-Amino-1-propanesulfonic acid site steady-state inactivation of Kv4.3 channels occurs predominantly from the closed state [21,22], here we determined whether acacetin would affect the development kinetics of closedstate inactivation of hKv4.3 channels. Acacetin (10 mM) slightly accelerated the closed-state inactivation of hKv4.3 channels (Figure S1). The closed-state inactivation time constant was 1683.36134.1 ms in control, and 1355.2659.2 ms in 10 mM acacetin (n = 6, P,0.05 vs. control). The result suggests that acacetin may accelerate the kinetics of closed-state inactivation of hKv4.3 channels.Molecular determinants of hKv4.3 channel blockade by acacetinThe molecular determinants of the blockade of hKv4.3 channels by acacetin were investigated using the mutants T366A and T367A in the P-loop helix, and V392A, I395A, and V399A in the S6 transmembrane domain. Figure 6A shows the representative current traces of wild type (WT), T366A, T367A, V392A, I395A, and V399A hKv4.3 channels activated with a 300-ms voltage step to +50 mV from a holding potential of 280 mV inAcacetin Blocks hKv4.3 ChannelsFigure 4. Effect of acacetin on recovery of hKv4.3 current from inactivation. A. Protocol and hKv4.3 current traces recorded in a representative cell before (control) and after 10 mM acacetin (8 min) used to assess the time constant of recovery of the channel from inactivation. B. Mean values of recovery time course of hKv4.3 current from inactivation were fitted to a mono-exponential function before and after application of 10 mM acacetin. doi:10.1371/journal.pone.0057864.gthe absence and presence of acacetin (30 mM). Acacetin at 30 mM markedly blocked the WT hKv4.3 current. Less inhibition was observed for the T366A, T367A, V392A, I395A, and V399A 
currents. The mean values of percentage inhibition of hKv4.3 currents are illustrated in Fig. 7B. Acacetin at 30 mM inhibited the WT hKv4.3 current by 74.263.3 (n = 12), T366A by 49.168.3 (n = 8, P,0.01 vs. WT), T367A by 54.966.5 (n 
= 7, P,0.01 vs. WT), V392A by 64.563.9 (n = 9, P,0.05 vs. WT), I395A by 65.662.7 (n = 8, P,0.05 vs. WT), and V399A by 62.963.7 (n = 6, P,0.05. vs. WT), respectively. The concentration-dependent response to acacetin was evaluated in WT and mutant hKv4.3 channels, and the resulting curves were fitted to a H.Ckade of hKv4.3 channels by acacetin was determined at 0.2, 1, 2, and 3.3 Hz using a train of 20 pulses of a 200-ms voltage step. Figure 5A shows the normalized hKv4.3 current traces recorded at 3.3 Hz in a representative cell before and after application of 3 mM acacetin. Though hKv4.3 current showed a significant use-dependent inhibition in control, the use-dependent blockade was evident with acacetin. The current blockade at the first pulse was less than that at following pulses. The fractional blockade of hKv4.3 current at each frequency with 3 mM acacetin is illustrated in Fig. 5B. The use-dependent blockade was enhanced as the depolarization frequency was increased. In addition, acacetin exhibited more potent of inhibition on hKv4.3 current at 3.3 Hz than that at 0.2 Hz and 1.0 Hz. Figure 5C shows the percentage current at the 20th pulse of different frequencies with 0.1?00 mM acacetin. The curves were fitted to a Hill equation to obtain the IC50. The IC50 of acacetin for inhibiting hKv4.3 current was reduced as increase of the depolarization frequency (IC50: 7.9, 6.1, 3.9, and 3.2 mM at 0.2, 1.0, 2.0, and 3.3 Hz, respectively).Effect of acacetin on closed-state inactivation of hKv4.3 currentThe steady-state inactivation of Kv4.3 channels occurs predominantly from the closed state [21,22], here we determined whether acacetin would affect the development kinetics of closedstate inactivation of hKv4.3 channels. Acacetin (10 mM) slightly accelerated the closed-state inactivation of hKv4.3 channels (Figure S1). The closed-state inactivation time constant was 1683.36134.1 ms in control, and 1355.2659.2 ms in 10 mM acacetin (n = 6, P,0.05 vs. control). The result suggests that acacetin may accelerate the kinetics of closed-state inactivation of hKv4.3 channels.Molecular determinants of hKv4.3 channel blockade by acacetinThe molecular determinants of the blockade of hKv4.3 channels by acacetin were investigated using the mutants T366A and T367A in the P-loop helix, and V392A, I395A, and V399A in the S6 transmembrane domain. Figure 6A shows the representative current traces of wild type (WT), T366A, T367A, V392A, I395A, and V399A hKv4.3 channels activated with a 300-ms voltage step to +50 mV from a holding potential of 280 mV inAcacetin Blocks hKv4.3 ChannelsFigure 4. Effect of acacetin on recovery of hKv4.3 current from inactivation. A. Protocol and hKv4.3 current traces recorded in a representative cell before (control) and after 10 mM acacetin (8 min) used to assess the time constant of recovery of the channel from inactivation. B. Mean values of recovery time course of hKv4.3 current from inactivation were fitted to a mono-exponential function before and after application of 10 mM acacetin. doi:10.1371/journal.pone.0057864.gthe absence and presence of acacetin (30 mM). Acacetin at 30 mM markedly blocked the WT hKv4.3 current. Less inhibition was observed for the T366A, T367A, V392A, I395A, and V399A currents. The mean values of percentage inhibition of hKv4.3 currents are illustrated in Fig. 7B. Acacetin at 30 mM inhibited the WT hKv4.3 current by 74.263.3 (n = 12), T366A by 49.168.3 (n = 8, P,0.01 vs. WT), T367A by 54.966.5 (n = 7, P,0.01 vs. WT), V392A by 64.563.9 (n = 9, P,0.05 vs. WT), I395A by 65.662.7 (n = 8, P,0.05 vs. WT), and V399A by 62.963.7 (n = 6, P,0.05. vs. WT), respectively. The concentration-dependent response to acacetin was evaluated in WT and mutant hKv4.3 channels, and the resulting curves were fitted to a H.
