title: Causes of excitation-induced muscle cell damage in isometric contractions: mechanical stress or calcium overload?
abstract: Prolonged or unaccustomed exercise leads to muscle cell membrane damage, detectable as release of the intracellular enzyme lactic acid dehydrogenase (LDH). This is correlated to excitation-induced influx of Ca2+, but it cannot be excluded that mechanical stress contributes to the damage. We here explore this question using N-benzyl-p-toluene sulfonamide (BTS), which specifically blocks muscle contraction. Extensor digitorum longus muscles were prepared from 4-wk-old rats and mounted on holders for isometric contractions. Muscles were stimulated intermittently at 40 Hz for 15-60 min or exposed to the Ca2+ ionophore A23187. Electrical stimulation increased 45Ca influx 3-5 fold. This was followed by a progressive release of LDH, which was correlated to the influx of Ca2+. BTS (50 microM) caused a 90% inhibition of contractile force but had no effect on the excitation-induced 45Ca influx. After stimulation, ATP and creatine phosphate levels were higher in BTS-treated muscles, most likely due to the cessation of ATP-utilization for cross-bridge cycling, indicating a better energy status of these muscles. No release of LDH was observed in BTS-treated muscles. However, when exposed to anoxia, electrical stimulation caused a marked increase in LDH release that was not suppressed by BTS but associated with a decrease in the content of ATP. Dynamic passive stretching caused no increase in muscle Ca2+ content and only a minor release of LDH, whereas treatment with A23187 markedly increased LDH release both in control and BTS-treated muscles. In conclusion, after isometric contractions, muscle cell membrane damage depends on Ca2+ influx and energy status and not on mechanical stress.
authors.affiliation: Department of Physiology and Biophysics, University of Aarhus, Denmark. af@fi.au.dk
authors.forename: Anne
authors.initials: A
authors.lastname: Fredsted
authors.forename: Hanne
authors.initials: H
authors.lastname: Gissel
authors.forename: Klavs
authors.initials: K
authors.lastname: Madsen
authors.forename: Torben
authors.initials: T
authors.lastname: Clausen
mesh_terms: D000255:Adenosine Triphosphate / Q000378:metabolism; D000818:Animals; D002118:Calcium / Q000378:metabolism*; D012090:Cumulative Trauma Disorders / Q000209:etiology* / Q000503:physiopathology*; D004558:Electric Stimulation / Q000009:adverse effects*; D005260:Female; D066298:In Vitro Techniques; D007537:Isometric Contraction; D008297:Male; D018482:Muscle, Skeletal / Q000293:injuries* / Q000503:physiopathology*; D051381:Rats; D017208:Rats, Wistar; D013314:Stress, Mechanical
chemical_list: D000255:Adenosine Triphosphate; D002118:Calcium