Attenuation of contraction-induced skeletal muscle injury by bromelain

The proteolytic enzyme, bromelain, reportedly has therapeutic effects in the treatment of inflammation and soft tissue injuries. We tested the hypothesis that bromelain attenuates skeletal muscle injury induced by lengthening contractions. The left extensor digitorum longus (EDL) muscle of anesthetized hamsters was injured using a motorized foot pedal which repeatedly flexed/extended the foot through a range of 125°. The EDL muscle was electrically stimulated for 400 ms during plantarflexion. Animals were assigned randomly to either a 0-d group (evaluated 3-h post-injury) or to untreated (UT) or bromelain-treated (T) groups, evaluated 3,7, or 14 d post-injury. Following injury, T received 5 mg-kg⁻¹ b.w. of bromelain, twice daily. Maximum isometric tetanic force (P_o) was measured in vitro, then muscles were fixed, sectioned, and examined for evidence of fiber damage. The P_o of injured muscles from T were higher than P_o of injured muscles from UT at 3 (18.7 ± 0.4 vs 16.5 ± N·cm⁻² and 14 d (20.5 ± 0.6 vs 18.2 ± 0.6 N-cnr²) (P < 0.05), but not 7 d (19.5 ± 0.7 vs 17.7 ± 0.8 N·cm⁻²). The P_o of UT injured muscles were significantly lower than P_o of contralateral control muscles at all time periods. P_o of injured muscles from T were lower than P_o from control muscles at 3 and 7 d (P < 0.05), but not 14 d. The number of intact fibers of 3-d UT injured muscles was lower than the number of intact fibers in control muscles (P < 0.05). No difference in fiber number between controls and the 3-d treated group was observed. Thus, daily oral bromelain treatments of 10 mg·kg⁻¹ attenuated the development of contraction-induced injury in hamster EDL muscles.