(A) Representative ClC-1 currents obtained from FDB fibers isolated from HSA^LR mice electroporated with either invert (left) or antisense (middle) morpholino and WT mice electroporated with antisense morpholino (right). The dashed lines represent the 0 current level. Capacitative currents recorded from each fiber are shown in the inset of each panel (scale bars: vertical, 3 nA; horizontal, 4 ms). Superimposed traces (solid lines) of normalized ClC-1 current deactivation at –100 mV in FDB fibers obtained from invert- (circles) and antisense-treated (squares) HSA^LR mice and antisense-treated WT mice (triangles) fit with a second-order exponential are shown in the inset to the left panel. Note that accelerated ClC-1 deactivation kinetics of FDB fibers obtained from HSA^LR mice are normalized only following treatment with antisense morpholino. (B) Membrane potential (Vm) dependence of average instantaneous ClC-1 current density recorded from FDB fibers of 16- to 18-day-old WT mice treated with invert morpholino (n = 11), WT mice treated with antisense morpholino (n = 10), HSA^LR mice treated with invert morpholino (n = 12), and HSA^LR mice treated with antisense morpholino (n = 16). (C) Average relative Po-Vm curves for the same experiments shown in B. Smooth curves through each data set were generated using a modified Boltzmann equation (10). (D) Average relative contribution of the fast (A_f/A_total), slow (A_s/A_total), and nondeactivating (C/A_total) components of ClC-1 current deactivation elicited from a voltage step to –100 mV for the same experiments shown in B. Mean ± SEM; *P < 0.05 invert-treated HSA^LR fibers compared with each of the other experimental conditions; t test. Myotonia was significantly reduced 3 (E) and 8 (F) weeks following injection of antisense morpholino. Mean ± SD; n = 3–7 per group. Antisense morpholino was injected into one TA; invert morpholino was injected into the contralateral TA; and gastrocnemius muscle served as an untreated control. **P < 0.0001 for antisense versus invert-treated control; ANOVA.