Apocynin treatment prevents loss of skeletal muscle function caused by CS exposure
Eight weeks of CS exposure resulted in a loss of skeletal muscle mass in mice similar to that observed in human smokers, as evidenced by a 10% reduction in gross weight of the tibialis anterior (TA) muscle (Figure 2A) which is a prime mover of the hind limb, predominated by fast-twitch myofibers. CS exposure also caused an ~8% reduction in the weight of soleus (Table 2) which is a slow-twitch fibers predominated muscle of the hind limbs (Timson, Bowlin, Dudenhoeffer & George, 1985), suggesting the muscle wasting effect of CS exposure is unbiased by fiber composition in our model. In addition to the loss of muscle mass, CS exposure also resulted in a significant reduction in contractile force (Figure 2B) and maximum contraction rate (Figure 2D) of the TA muscles which translated to a 54% decrease in specific force generated (Figure 2C), suggesting CS exposure caused muscle weakness. In addition to preventing the loss of TA mass, apocynin treatment attenuated the CS-induced skeletal muscle weakness as evidenced by the improved contractile force, maximum contraction rate and specific force (Figure 2A-D).
We next conducted qPCR analyses to examine the molecular changes within the TA muscles. In line with the loss of mass and function, CS exposure resulted in a 50% reduction in Igf1-eb (a precursor isoform of muscle-derived IGF-1; Figure 2E) and a 2-fold increase in Mstn(myostatin; Figure 2F) expression which were completely prevented by apocynin treatment. Tnfα expression remained unaltered regardless of CS exposure or apocynin treatment, suggesting the CS-induced phenotypical and molecular changes are unlikely to involve myocellular inflammation. Lastly, our oxyblot analysis revealed a 2.3-fold increase in protein carbonylation of TA muscles following CS exposure which was completely prevented by apocynin treatment (Figure 2 H-I), suggesting the protective effects of apocynin in vivo may be related to its ability to antagonize the oxidative burden evoked by CS exposure.