OBJECTIVE Myotonia is caused by involuntary firing of skeletal muscle action potentials and causes debilitating stiffness. Current treatments are insufficiently efficacious and associated with side effects. Myotonia can be triggered by voluntary movement (electrically-induced myotonia) or percussion (mechanically-induced myotonia). Whether distinct molecular mechanisms underlie these triggers is unknown. Our goal was to identify ion channels involved in mechanically-induced myotonia and to evaluate block of the channels involved as a novel approach to therapy. METHODS We developed a novel system to enable study of mechanically-induced myotonia using both genetic and pharmacologic mouse models of myotonia congenita. We extended ex vivo studies of excitability to in vivo studies of muscle stiffness. RESULTS As previous work suggests activation of transient receptor potential vanilloid 4 (TRPV4) channels by mechanical stimuli in muscle, we examined the role of this cation channel. Mechanically-induced myotonia was markedly suppressed in TRPV4-null muscles and in muscles treated with TRPV4 small molecule antagonists. The suppression of mechanically-induced myotonia occurred without altering intrinsic muscle excitability such that myotonia triggered by firing of action potentials (electrically-induced myotonia) was unaffected. When injected intraperitoneally, TRPV4 antagonists lessened the severity of myotonia in vivo by approximately 80%. INTERPRETATION These data demonstrate for the first time that there are distinct molecular mechanisms triggering electrically- and mechanically-induced myotonia. Our data indicates that activation of TRPV4 during muscle contraction plays an important role in triggering myotonia in vivo. Elimination of mechanically-induced myotonia by TRPV4 inhibition offers a new approach to treating myotonia. This article is protected by copyright. All rights reserved.
TRPV4 antagonism prevents mechanically-induced myotonia.