In vivo intracellular Ca2+ profiles after eccentric rat muscle contractions: Addressing the mechanistic bases for repeated bout protection.

Eccentric contractions (ECC) are accompanied by accumulation of intracellular calcium ions ([Ca²⁺]_i) and induce skeletal muscle damage. Suppressed muscle damage in repeated bouts of ECC is well characterized, however, whether it is mediated by altered Ca²⁺ profiles remains unknown. PURPOSE: We tested the hypothesis that repeated ECC suppresses Ca²⁺ accumulation via adaptions in Ca²⁺ regulation. METHODS: 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 Ca²⁺ indicator Fura-2 AM and the 340/380 nm ratio was evaluated as [Ca²⁺]_i. Ca²⁺ handling proteins were measured by western blots. RESULTS: ECC induced [Ca²⁺]_i increase in both groups, but ECC-RB evinced a markedly suppressed [Ca²⁺]_i (Time: P < 0.01, Group: P = 0.0357). 5 hours post-ECC, in contrast to the localized [Ca²⁺]_i accumulation in ECC-SB, ECC-RB exhibited lower and more uniform [Ca²⁺]_i (P < 0.01). In ECC-RB mitochondria Ca²⁺ uniporter complex components, MCU and MICU2, were significantly increased pre-second ECC bout (P < 0.01) and both SERCA1 and MICU1 were better preserved after contractions (P < 0.01). CONCLUSION: 14 days after novel ECC skeletal muscle mitochondrial Ca²⁺ regulating proteins were elevated. Following subsequent ECC [Ca²⁺]i 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 SR Ca²⁺ regulation.