Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disorder, characterised by a loss of motor neurons, leading to progressive muscular atrophy. Mutations in the gene encoding superoxide dismutase (SOD1) have been indicated to underlie the disease in a subgroup of patients. In accordance, transgenic mice over-expressing a mutant human SOD1 gene, develop motor neuron loss. This study was undertaken to evaluate in vitro the contractile and metabolic properties of selected hindlimb muscles in transgenic ALS mice.
Male mice overexpressing a mutant human SOD1 containing a glycine93 ┌ alanine substitution (G93A), or a non-mutant human SOD1 (wild-type, WT) were bred. Seven G93A and seven WT mice, aged 115 days, were anaesthetised by intraperitoneal infusion of pento-barbitone sodium (50 mg kg-1 b.w.) and slow-twitch soleus and fast-twitch extensor digitorum longus (EDL) muscles were carefully dissected out and mounted in between electrodes in oxygenated Krebs solution. Maximal tension, time-to-peak tension and relaxation time were evaluated in electrically induced twitch contractions, as well as maximal tetanus tension and fatigability during repetitive tetani. Contralateral muscles were excised for biochemical determination of resting ATP, creatine, phosphocreatine and glycogen content. Groups were compared by Student’s t test and data are given as means ± S.E.M. Animals were killed using an overdose of anaesthetic and the experiments were approved by the local ethics committee.
No difference was observed in maximal twitch tension between WT and G93A for both muscles. However, in EDL, but not in soleus, twitch time-to-peak tension and relaxation time were increased (P < 0.05) in G93A compared with WT. Similarly, in EDL, but not in soleus, G93A muscles showed reduced ATP and total creatine contents (P < 0.05). Furthermore, maximal tetanic tension was reduced by 35 % and 20 % in EDL and soleus, respectively (P < 0.05). Soleus and EDL muscles were both more fatigue resistant in G93A compared with WT, and showed faster recovery from fatigue (P < 0.05). Muscle glycogen content (mean ± S.E.M.) in WT and G93A, respectively, was 28 ± 2 and 41 ± 7 µmol (g dw)-1 for soleus (P < 0.05) and 60 ± 11 and 89 ± 11 µmol (g dw)-1 for EDL (P < 0.05).
EDL muscles of G93A mice exhibited slower tension development and relaxation, which is presumably caused by a reduction in high-energy phosphate content. In contrast, contractile properties of the soleus muscle are hardly affected at this stage of the disease. This could point to a higher vulnerability of motoneurons innervating fast-twitch fibres to degeneration. G93A mice have higher muscle glycogen content, which improves fatigue resistance during prolonged electrical stimulation.