Effects of Hyperbaric Oxygen at 1.25 Atmospheres Absolute with Normal Air on Macrophage Number and Infiltration during Rat Skeletal Muscle Regeneration

Use of mild hyperbaric oxygen less than 2 atmospheres absolute (2026.54 hPa)with normal air is emerging as a common complementary treatment for severemuscle injury. Although hyperbaric oxygen at over 2 atmospheres absolute with100% O2 promotes healing of skeletal muscle injury, it is not clear whether mildhyperbaric oxygen is equally effective. The purpose of the present study was toinvestigate the impact of hyperbaric oxygen at 1.25 atmospheres absolute (1266.59hPa) with normal air on muscle regeneration. The tibialis anterior muscle of maleWistar rats was injured by injection of bupivacaine hydrochloride, and rats wererandomly assigned to a hyperbaric oxygen experimental group or to a non hyperbaric oxygen control group. Immediately after the injection, rats were exposed to hyperbaric oxygen, and the treatment was continued for 28 days. The cross sectional area of centrally nucleated muscle fibers was significantly larger in rats exposed to hyperbaric oxygen than in controls 5 and 7 days after injury. The number of CD68- or CD68- and CD206-positive cells was significantly higher in rats exposed to hyperbaric oxygen than in controls 24 h after injury. Additionally, tumor necrosis factor-a and interleukin-10 mRNA expression levels were significantlyhigher in rats exposed to hyperbaric oxygen than in controls 24 h after injury. The number of Pax7- and MyoD- or MyoD- and myogenin-positive nuclei per mm2 and the expression levels of these proteins were significantly higher in rats exposed tohyperbaric oxygen than in controls 5 days after injury. These results suggest that mild hyperbaric oxygen promotes skeletal muscle regeneration in the early phase after injury, possibly due to reduced hypoxic conditions leading to accelerated macrophage infiltration and phenotype transition. In conclusion, mild hyperbaricoxygen less than 2 atmospheres absolute with normal air is an appropriate support therapy for severe muscle injuries.