Physiologists at the University of California are doing fundamental research on a new way of building up more muscle mass. Not by providing the muscles with more amino acids or growth factors, not by giving them better training regimes – but by getting the immune system to build up muscle tissue. In 2007 the researchers demonstrated that some immune cells, known as macrophages, induce muscle cells to grow when they are subject to heavy exertion.
Macrophages are simple immune cells. They are found in blood and tissues, and become active when they notice things that do not belong in the body, such as bacteria or a virus. Macrophages consume the intruders and destroy them. It is also known that they play a role in muscle growth. After intensive physical exertion they appear in large numbers in the muscles that were trained. And what happens next? That’s what we don’t know.
The researchers tried to fill the gap by doing an animal study. They took mice and prevented them from using one of their hind legs for the first 10 days. That’s called unloading. As a result the calf muscle [soleus] loses about 40 percent of its mass. Then the researchers let the mice use their hind leg again. Physiologists call this re-loading. When the mice started to use the muscles in the leg again, not only did the muscles grow, but inflammatory processes also occurred in which macrophages from the body accumulated in the leg muscles.
Half of the experimental animals were given anti-F4/80. This deactivates most macrophages. The other half were not given this [control]. The researchers studied the changes that took place in the rats’ muscles during the first 2-4 days of reloading. The figures here summarise the most important findings.
In the top graph you can see that more macrophages accumulated in the soleus of the control group mice during reloading. In the second graph you see that the number of damaged muscle fibres decreases in the control group, but increases in the anti-F4/80 group.
The graph below shows that the number of new muscles cells in the regenerating tissue increases in the control group, but not in the mice in which the macrophages had been deactivated.
The graph at the bottom shows the net results of all the effects put together. The muscle fibre grows in the control group, but not in the anti-F4/80 group.
“These findings highlight the potential value of targeting macrophages for modulating the repair and growth of skeletal muscle”, the researchers conclude.
Extracts of Echinacea purpurea and Cat’s claw activate the immune system. Could you use these herbs to grow faster?
Macrophages promote muscle membrane repair and muscle fibre growth and regeneration during modified muscle loading in mice in vivo
Muscle injury or modified muscle use can stimulate muscle invasion by leucocytes that have the potential to increase tissue damage or promote tissue growth and repair. In the present investigation, we examined the role of macrophages in muscle injury, repair and regeneration during modified muscle loading. Weight-bearing was removed from the hindlimbs of mice for 10 days followed by reloading through normal ambulation. During the unloading period, soleus muscle fibre cross-section decreased by 38%. Prior to the onset of reloading, mice received a series of intraperitoneal injections of anti-F4/80, which binds a mouse macrophage surface antigen. Although anti-F4/80 injections did not affect macrophage numbers in soleus muscles at 2 days of reloading, macrophages were reduced by 86% at 4 days of reloading. Muscle membrane lysis during the reloading period did not differ at 2 days of reloading between anti-F4/80-treated mice and mice that received isotype control antibody. However, control animals showed large decreases in the number of fibres with membrane lesions at 4 days of reloading, but this membrane repair did not occur in macrophage-depleted mice. Macrophage-depletion also reduced muscle regeneration (indicated by central nucleation) and satellite cell differentiation (indicated by reductions in MyoD-expressing satellite cells) and prevented growth of muscle fibres that normally occurred in control animals between days 2 and 4 of reloading. These findings collectively show that macrophages play a significant role in muscle fibre membrane repair, regeneration and growth during increased muscle use after a period of atrophy.