BK channels promote action potential repolarization in skeletal muscle but contribute little to myotonia

Patients with myotonia congenita suffer from slowed relaxation of muscle (myotonia), due to hyperexcitability caused by loss-of-function mutations in the ClC-1 chloride channel. A recent study suggested that block of large-conductance voltage- and Ca²⁺- activated K⁺ channels (BK) may be effective as therapy. The mechanism underlying efficacy was suggested to be lessening of the depolarizing effect of build-up of K⁺ in t-tubules of muscle during repetitive firing. BK channels are widely expressed in the nervous system and have been shown to play a central role in regulation of excitability, but their contribution to muscle excitability has not been determined. We performed intracellular recordings as well as force measurements in both wild type and BK^-/- mouse extensor digitorum longus muscles. Action potential width was increased in BK^-/- muscle due to slowing of repolarization, consistent with the possibility K⁺ build-up in t-tubules is lessened by block of BK channels in myotonic muscle. However, there was no difference in the severity of myotonia triggered by block of muscle Cl^- channels with 9-anthracenecarboxylic acid (9AC) in wild type and BK^-/- muscle fibers. Further study revealed no difference in the interspike membrane potential during repetitive firing suggesting there was no reduction in K⁺ build-up in t-tubules of BK^-/- muscle. Force recordings following block of muscle Cl^- channels demonstrated little reduction in myotonia in BK^-/- muscle. In contrast, the current standard of care, mexiletine, significantly reduced myotonia. Our data suggest BK channels regulate muscle excitability, but are not an attractive target for therapy of myotonia.