In vivo intracellular Ca profiles after eccentric rat muscle contractions: addressing the mechanistic bases for repeated bout protection.

Eccentric contractions (ECC) are accompanied by the accumulation of intracellular calcium ions ([Ca]) and induce skeletal muscle damage. Suppressed muscle damage in repeated bouts of ECC is well characterized; however, whether it is mediated by altered Ca profiles remains unknown. We tested the hypothesis that repeated ECC suppresses Ca accumulation via adaptations in Ca regulation. Male Wistar rats were divided into two groups: ECC single bout (ECC-SB) and repeated bout (ECC-RB). Tibialis anterior (TA) muscles were subjected to ECC (40 times, 5 sets) once (ECC-SB) or twice 14 days apart (ECC-RB). Under anesthesia, the TA muscle was loaded with Ca indicator Fura 2-AM, and the 340/380 nm ratio was evaluated as [Ca]. Ca handling proteins were measured by Western blots. ECC induced [Ca] increase in both groups, but ECC-RB evinced a markedly suppressed [Ca] (Time: < 0.01, Group: = 0.0357). Five hours post-ECC, in contrast to the localized [Ca] accumulation in ECC-SB, ECC-RB exhibited lower and more uniform [Ca] ( < 0.01). In ECC-RB, mitochondria Ca uniporter complex (MCU) components MCU and MICU2 were significantly increased pre-second ECC bout ( < 0.01), and both SERCA1 and MICU1 were better preserved after contractions ( < 0.01). Fourteen days after novel ECC, skeletal muscle mitochondrial Ca regulating proteins were elevated. Following subsequent ECC, [Ca] accumulation and muscle damage were suppressed and SERCA1 and MICU1 preserved. These findings suggest that tolerance to a subsequent ECC bout is driven, at least in part, by enhanced mitochondrial and sarcoplasmic reticulum Ca regulation. We demonstrated a reduced [Ca] profile with suppressed muscle damage after a repeated bout of ECC in vivo: the ECC-induced immediate [Ca] increase was suppressed and the persistence of increased [Ca] with localized accumulation was diminished after repeated ECC. This effect occurred consonant with the upregulation of the mitochondrial Ca uniporter complex and better preservation of SERCA1 and MICU1. These findings suggest that the mechanistic bases for repeated bout protection involve adaptation of Ca regulation.