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Exercise and suspension hypokinesia-induced alterations in mechanical properties of rat fast and slow-twitch skeletal muscles.

Physical activity has a modulatory role on regulatory steps of excitation-contraction coupling (ECC) determining skeletal muscle contractility. We evaluated and compared the contractile responsiveness and caffeine-induced contractures of fast (extensor digitorum longus; EDL) and slow-twitch (soleus; SOL) muscles in suspension hypokinesia (SH) and exercised rats. After SH or low intensity exercise, EDL and SOL were isolated, twitch and tetanic contractions and caffeine (10 mM) contractures were recorded. Twitch and tetanic contractions of EDL increased by 60% in exercised rats (p <0.05) while no alteration was observed after SH. Exercise did not alter twitch and tetanic contractions of SOL, while SH depressed contractions (p <0.05). Caffeine contractures were diminished in exercised rat EDL (P <0.05). In SH-rat EDL, contractures increased in amplitude (p <0.01) with a rapid time course (p <0.05). Contractures did not change in SOL after exercise or SH. We concluded that SH and exercise exerted diverse modulatory effects on skeletal muscle contractility. Contractile improvement due to exercise was prominent in EDL. Our results suggest that the muscle-type specific adaptations are related to a change in ECC due to the differences in the regulatory steps, particularly in the intracellular Ca(2+) handling mechanisms.