title: In vivo evaluation of antimyotonic efficacy of β-adrenergic drugs in a rat model of myotonia.
abstract: The sodium channel blocker mexiletine is considered the first-line drug in myotonic syndromes, a group of muscle disorders characterized by membrane over-excitability. We previously showed that the β-adrenoceptor modulators, clenbuterol and propranolol, block voltage-gated sodium channels in a manner reminiscent to mexiletine, whereas salbutamol and nadolol do not. We now developed a pharmacological rat model of myotonia congenita to perform in vivo preclinical test of antimyotonic drugs. Myotonia was induced by i.p. injection of 30 mg/kg of anthracene-9-carboxylic acid (9-AC), a muscle chloride channel blocker, and evaluated by measuring the time of righting reflex (TRR). The TRR was prolonged from <0.5 s in control conditions to a maximum of ∼4 s, thirty minutes after 9-AC injection, then gradually recovered in a few hours. Oral administration of mexiletine twenty minutes after 9-AC injection significantly hampered the TRR prolongation, with an half-maximum efficient dose (ED(50)) of 12 mg/kg. Both propranolol and clenbuterol produced a dose-dependent antimyotonic effect similar to mexiletine, with ED(50) values close to 20 mg/kg. Antimyotonic effects of 40 mg/kg mexiletine and propranolol lasted for 2 h. We also demonstrated, using patch-clamp methods, that both propranolol enantiomers exerted a similar block of skeletal muscle hNav1.4 channels expressed in HEK293 cells. The two enantiomers (15 mg/kg) also showed a similar antimyotonic activity in vivo in the myotonic rat. Among the drugs tested, the R(+)-enantiomer of propranolol may merit further investigation in humans, because it exerts antimyotonic effect in the rat model, while lacking of significant activity on the β-adrenergic pathway. This study provides a new and useful in vivo preclinical model of myotonia congenita in order to individuate the most promising antimyotonic drugs to be tested in humans.
authors.affiliation: Section of Pharmacology, Department of Pharmacy, University of Bari-Aldo Moro, Via Orabona 4 - Campus, 70125 Bari, Italy. jfdesaphy@farmbiol.uniba.it
authors.forename: Jean-François
authors.initials: JF
authors.lastname: Desaphy
authors.forename: Teresa
authors.initials: T
authors.lastname: Costanza
authors.forename: Roberta
authors.initials: R
authors.lastname: Carbonara
authors.forename: Diana
authors.initials: D
authors.lastname: Conte Camerino
mesh_terms: D000318:Adrenergic beta-Agonists / Q000627:therapeutic use*; D000319:Adrenergic beta-Antagonists / Q000627:therapeutic use*; D000818:Animals; D004195:Disease Models, Animal; D004353:Drug Evaluation, Preclinical / Q000379:methods; D057809:HEK293 Cells; D006801:Humans; D009222:Myotonia / Q000188:drug therapy* / Q000503:physiopathology; D011897:Random Allocation; D051381:Rats; D017208:Rats, Wistar; D016896:Treatment Outcome
chemical_list: D000318:Adrenergic beta-Agonists; D000319:Adrenergic beta-Antagonists