Sodium channel slow inactivation as a therapeutic target for myotonia congenita

Abstract

Objective: Patients with myotonia congenita have muscle hyperexcitability due to loss-of-function mutations in the chloride channel in skeletal muscle, which causes spontaneous firing of muscle action potentials (myotonia), producing muscle stiffness. In patients, muscle stiffness lessens with exercise, a change known as the warmup phenomenon. Our goal was to identify the mechanism underlying warmup and to use this information to guide development of novel therapy.

Methods: To determine the mechanism underlying warmup, we used a recently discovered drug to eliminate muscle contraction, thus allowing prolonged intracellular recording from individual muscle fibers during induction of warmup in a mouse model of myotonia congenita.

Results: Changes in action potentials suggested slow inactivation of sodium channels as an important contributor to warmup. These data suggested that enhancing slow inactivation of sodium channels might offer effective therapy for myotonia. Lacosamide and ranolazine enhance slow inactivation of sodium channels and are approved by the US Food and Drug Administration for other uses in patients. We compared the efficacy of both drugs to mexiletine, a sodium channel blocker currently used to treat myotonia. In vitro studies suggested that both lacosamide and ranolazine were superior to mexiletine. However, in vivo studies in a mouse model of myotonia congenita suggested that side effects could limit the efficacy of lacosamide. Ranolazine produced fewer side effects and was as effective as mexiletine at a dose that produced none of mexiletine's hypoexcitability side effects.

Interpretation: We conclude that ranolazine has excellent therapeutic potential for treatment of patients with myotonia congenita.

© 2014 American Neurological Association.

Figures

Figure 1

Induction of reversible warm-up during intracellular recording: Shown on the left is the response of a normal skeletal muscle fiber to a 60 ms injection of depolarizing current. The fiber is able to repeatedly fire action potentials during the current injection, but firing stops immediately after termination of current injection. The three traces on the right are from an individual ClCadr muscle fiber at baseline, after warm-up, and 5 minutes after warm-up. At baseline the ClCadr fiber is hyperexcitable and continues to fire action potentials after termination of the current injection. After 5000 action potentials have been triggered to induce warm-up, excitability of the fiber has normalized such that the no action potentials are fired after termination of current injection. Following 5 minutes of rest, hyperexcitability has returned such that action potentials continue to be fired after termination of current injection. AP = action potential, min = minutes.

Figure 2

Alteration of the action potential waveform induced by warm-up: Shown are three superimposed action potential traces from an individual muscle fiber before and after 5000 action potentials, and again after 5 minutes of inactivity. Following 5000 action potentials, there is slight hyperpolarization of resting potential, slight elevation of threshold, reduction in both rate of rise and peak of the action potential, as well as slowing of repolarization. Following 5 minutes of inactivity, the action potential has recovered to closely resemble its initial waveform.

Figure 3

Simulation of action potentials following warm-up is consistent with reduction of voltage gated Na and K conductance as well as a depolarized shift in K equilibrium potential. A) Shown superimposed are an action potential recorded from a ClCadr muscle fiber (black, baseline) and a simulated action potential (red). In the simulated fiber, 15nA of current injection was required to reach threshold. B) Shown are superimposed action potential traces from an individual muscle fiber before (black, same trace as in A) and immediately after 5000 action potentials to induce warm-up (red). C–F: Shown in black in all 4 traces is the same simulated action potential shown in red in A. C) Superimposed on the simulated baseline action potential trace is the simulated action potential when leak conductance is increased by 15%. The traces are so similar that the red trace almost completely obscures the black trace. D) Superimposed (in red) on the baseline simulated trace is the simulated trace when GNa is reduced by 40%. E) Superimposed (in red) on the simulated baseline trace is the trace resulting from a 40% reduction in GNa and a 70% reduction in GK. F) Superimposed (in red) is the trace resulting from a 40% reduction in GNa, a 70% reduction in GK and a 5mV depolarization of EK.

Figure 4

The duration of warm-up depends on the duration of stimulation. Shown is a plot of the percent of fibers in which myotonia returned at various times following termination of stimulation at 20 Hz. AP = action potential.

Fig 5

Lacosamide and ranolazine are more effective than mexiletine in normalizing excitability of ClCadr muscle. A) Shown are the responses of 3 different ClCadr muscle fibers to a 60ms injection of depolarizing current following treatment with mexiletine. In the trace on the left, the fiber fired normally during current injection, but was hyperexcitable and fired an additional action potential following termination of current injection. In the trace in the middle, the fiber had normal excitability and was able to fire repetitively during current injection, but immediately stopped firing after termination of current injection. In the trace on the right, the fiber was hypoexcitable and unable to fire repetitively during the 60 ms current injection. B) Plotted for each drug is the percent of hyperexcitable (black), normally excitable (dark grey) and hypoexcitable (light grey) fibers for each dose of drug tested. The bar graph for each drug dose is based on at least 22 fibers from 3 different animals.

Figure 6

Mexiletine and ranolazine cause greater improvement in motor function than lacosamide. Shown on the left is a plot of the improvement in time of the righting reflex in mice 5 to 15 minutes following intraperitoneal injection of the doses indicated of mexiletine, lacosamide and ranolazine. The highest dose of all three drugs led to statistically significant improvement relative to saline injection (mexiletine p