title: Effect of phenytoin on the clinical signs and in vitro muscle twitch characteristics in horses with chronic intermittent rhabdomyolysis and myotonia.
abstract: In vitro twitch characteristics of the semimembranosus muscle were evaluated in 9 clinically normal horses, in 15 horses with chronic intermittent rhabdomyolysis (CIR) and in 2 horses with myotonia. Effects of phenytoin on in vitro muscle twitch and clinical signs of CIR and myotonia were evaluated in these same horses. Times to 90% relaxation were prolonged in the horses with CIR (mean +/- SEM, 186 +/- 5.9 ms) and in 2 horses with myotonia (197 and 177 ms) compared with those in clinically normal horses (mean +/- SEM, 146 +/- 2.1 ms). Horses with CIR also had significantly (P less than 0.05) longer times to 50% relaxation, compared with clinically normal horses. In the group of horses with CIR, Standardbreds had significantly (P less than 0.05) longer times to 90% and 50% relaxation, compared with Thoroughbreds. Times to 100% peak tension did not differ among the groups. Administration of phenytoin directly into a muscle preparation bath solution had no effect on muscle twitch properties. After the initial muscle biopsy, phenytoin was administered orally for 7 to 10 days to 4 horses with CIR, 2 myotonic horses, and 2 clinically normal horses before repeat biopsy from the same site in the contralateral semimembranosus muscle. Times to 90% relaxation decreased from 197 and 177 ms to 144 and 126 ms, respectively, in the 2 myotonic horses, from a mean of 192 (+/- 9) ms to 170 (+/- 9) ms in the 4 horses with CIR and remained unchanged (154 and 140 ms before vs 155 and 139 ms after treatment) in the 2 clinically normal horses.(ABSTRACT TRUNCATED AT 250 WORDS)
authors.affiliation: Section of Medicine, School of Veterinary Medicine, University of Pennsylvania, New Bolton Center, Kennett Square 19348.
authors.forename: J
authors.initials: J
authors.lastname: Beech
authors.forename: J E
authors.initials: JE
authors.lastname: Fletcher
authors.forename: F
authors.initials: F
authors.lastname: Lizzo
authors.forename: J
authors.initials: J
authors.lastname: Johnston
mesh_terms: D000284:Administration, Oral; D000818:Animals; D002908:Chronic Disease; D003402:Creatine Kinase / Q000032:analysis; D005260:Female; D005684:Gait; D006734:Horse Diseases / Q000188:drug therapy*; D006736:Horses / Q000097:blood / Q000502:physiology*; D066298:In Vitro Techniques; D008297:Male; D009119:Muscle Contraction / Q000187:drug effects*; D009222:Myotonia / Q000188:drug therapy / Q000503:physiopathology / Q000662:veterinary*; D010672:Phenytoin / Q000008:administration & dosage / Q000627:therapeutic use*; D012206:Rhabdomyolysis / Q000188:drug therapy / Q000503:physiopathology / Q000662:veterinary*
chemical_list: D010672:Phenytoin; D003402:Creatine Kinase