Enhanced activity of eccentric contraction induces alterations in in vitro sarcoplasmic reticulum Ca²⁺ handling in rat hindlimb muscles

Abstract

In order to investigate factors that contribute to eccentric contraction (ECC)-induced loss of tetanic force, Ca²⁺ uptake, Ca²⁺ release, and Ca²⁺-ATPase activity of the sarcoplasmic reticulum (SR) and sarcolemmal Na⁺-K⁺-ATPase activity were examined in rat fast-twitch skeletal muscles that underwent in situ ECC or isometric contraction (ISC) for up to 500 repetitions. The tetanic force at 60 Hz was more depressed in ECC-treated muscles than in ISC-treated muscles. SR Ca²⁺-ATPase activity displayed biphasic changes in response to ECC; after a temporary increase (up to 200 ECC repetitions), the activity decreased. With ECC, SR Ca²⁺ release rate and Na⁺-K⁺-ATPase activity decreased during the first 100 repetitions and remained almost constant thereafter. In contrast, the investigated variables (Ca²⁺-ATPase activity, Ca²⁺ release rate, and Na⁺-K⁺-ATPase activity) were unaltered in ISC-treated muscles. These results indicate that a more pronounced reduction in force output in ECC-treated muscles than in ISC-treated muscles might be attributable, at least in part, to impaired function of the SR Ca²⁺ release channel and/or Na⁺-K⁺-ATPase.