Abstract
To examine mechanisms underlying force reduction after the onset of chronic low-frequency (10 Hz) stimulation (CLFS), rabbit tibialis anterior muscles were exposed to various durations of CLFS (1 d, 6 d, 10 d, and 20 d). Isometric contractile properties and electromyographic (EMG) activity were studied in stimulated and contralateral muscles during a 10 min test at 10 Hz. In addition, activities and protein amounts of the sarcoplasmic reticulum Ca2+-ATPase and concentration of Na+,K+-ATPase were examined. Force output in the 1 d stimulated muscle had declined by 72% and EMG activity by 98%, suggesting refractoriness of a large population of fibres. Although twitch force and EMG gradually recovered in muscles stimulated for 6 d and longer, the muscles exhibited profoundly altered properties, i.e., enhanced fatigue resistance, absence of twitch potentiation, and prolonged contraction and relaxation times. These changes were associated with significant increases in Na+,K+-ATPase concentration (214%↑) and significant decreases in Ca2+-ATPase activity (65%↓) and concentration (70%↓) after 20 d of CLFS. Alterations in excitability and Ca2+ handling occur prior to changes in myofibrillar protein isoform expression, and may thus be responsible for the early alterations in functional characteristics.
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