Photobiomodulation therapy combined with static magnetic field preconditioning attenuates oxidative stress and preserves cell viability in C2C12 myoblasts: A hormetic dose-response study.

Photobiomodulation therapy combined with static magnetic field (PBMT-sMF) has demonstrated beneficial effects on skeletal muscle cells under oxidative stress. However, its potential as a preconditioning strategy - applied prior to an oxidative challenge - remains poorly investigated at the cellular level. This study aimed to evaluate the cytoprotective effects of PBMT-sMF preconditioning in C2C12 myoblasts under oxidative stress induced by hydrogen peroxide (H₂O₂). Cells were irradiated at three energy doses (9, 30, and 60 J) and, after a 3-h interval, subjected to 700 μM H₂O₂ for 1 h. Cell viability, nitric oxide (NO) production, reactive oxygen species (ROS) generation, and extracellular double-stranded DNA (dsDNA) were assessed. Three composite indices - the Cytoprotective Index (CPI), ROS Production per Viable Cell (RPVC), and NO Production per Viable Cell (NOPVC) - were calculated to provide an integrated interpretation of the dose-dependent cytoprotective response. H₂O₂ significantly reduced cell viability and elevated both ROS and NO production. Preconditioning with 9 J fully restored cell viability, normalized ROS to control levels, and reduced NO production below basal values, with comprehensive protection confirmed across all three composite indices. The 30 J dose conferred partial protection, with significant reductions in ROS and NO production and partial restoration of cell viability, whereas 60 J not only failed to protect but, paradoxically, worsened the oxidative and nitrosative burden per surviving cell. No significant differences in dsDNA levels were observed across groups. These findings demonstrate that PBMT-sMF preconditioning exerts dose-dependent cytoprotective effects consistent with a hormetic dose-response pattern, with 9 J emerging as the most effective dose among the three tested (9, 30, and 60 J). The temporal interval between irradiation and oxidative challenge is identified as a biologically active parameter, contributing to a more rational design of prophylactic photobiomodulation protocols.