title: Extracellular magnesium and calcium reduce myotonia in ClC-1 inhibited rat muscle.
abstract: Loss-of-function mutations in the ClC-1 Cl(-) channel trigger skeletal muscle hyperexcitability in myotonia congenita. For reasons that remain unclear, the severity of the myotonic symptoms can vary markedly even among patients with identical ClC-1 mutations, and may become exacerbated during pregnancy and with diuretic treatment. Since both these conditions are associated with hypomagnesemia and hypocalcemia, we explored whether extracellular Mg(2+) and Ca(2+) ([Mg(2+)]o and [Ca(2+)]o) can affect myotonia. Experimental myotonia was induced in isolated rat muscles by ClC-1 inhibition and effects of [Mg(2+)]o or [Ca(2+)]o on myotonic contractions were determined. Both cations dampened myotonia within their physiological concentration ranges. Thus, myotonic contractile activity was 6-fold larger at 0.3 than at 1.2 mM [Mg(2+)]o and 82-fold larger at 0.3 than at 1.27 mM [Ca(2+)]o. In intracellular recordings of action potentials, the threshold for action potential excitation was raised by 4-6 mV when [Mg(2+)]o was elevated from 0.6 to 3 mM, compatible with an increase in the depolarization of the membrane potential necessary to activate the Na(+) channels. Supporting this notion, mathematical simulations showed that myotonia went from appearing with normal Cl(-) channel function to disappearing in the absence of Cl(-) channel function when Na(+) channel activation was depolarized by 6 mV. In conclusion, variation in serum Mg(2+) and Ca(2+) may contribute to phenotypic variation in myotonia congenita patients.
authors.affiliation: Department of Biomedicine, Aarhus University, Ole Worms Allé 4, 8000 Aarhus C, Denmark.
authors.forename: Martin
authors.initials: M
authors.lastname: Skov
authors.forename: Anders
authors.initials: A
authors.lastname: Riisager
authors.forename: James A
authors.initials: JA
authors.lastname: Fraser
authors.forename: Ole B
authors.initials: OB
authors.lastname: Nielsen
authors.forename: Thomas H
authors.initials: TH
authors.lastname: Pedersen
mesh_terms: D000200:Action Potentials / Q000502:physiology; D000818:Animals; D002118:Calcium / Q000097:blood*; D018118:Chloride Channels / Q000378:metabolism*; D005260:Female; D008274:Magnesium / Q000097:blood*; D008297:Male; D008564:Membrane Potentials / Q000235:genetics; D018482:Muscle, Skeletal / Q000378:metabolism / Q000503:physiopathology*; D009154:Mutation / Q000235:genetics*; D009224:Myotonia Congenita / Q000235:genetics*; D051381:Rats; D017208:Rats, Wistar
chemical_list: C086321:CLC-1 channel; D018118:Chloride Channels; D008274:Magnesium; D002118:Calcium