A new study in mice shows that exercise causes muscle to release a peptide that builds the muscle's capacity for energy production and increases physical endurance, allowing for longer and more intense exercise.

The findings establish that the peptide, called musclin, is an "exercise factor" -- a hormone-like substance made by skeletal muscle in response to exercise and released into the bloodstream. The study shows that increased levels of circulating musclin trigger a signaling cascade that improves muscle performance and promotes production of mitochondria in muscle cells. The study was published in the journal Proceedings of the National Academy of Sciences.

The scientists used genetic engineering to make mice that don't have musclin. Although these animals look and act like wild type mice, they have lower exercise tolerance and are not able to exercise as long or as hard as wild type mice. However, infusing the musclin peptide back into these modified mice allows the animals to regain normal exercise capacity.

The researchers also showed that infusion of wild type mice with musclin increased the animals' voluntary treadmill activity; the mice ran faster and longer on the treadmill than wild type mice that received a placebo infusion of saline.

Further investigation showed that musclin signaling promotes production of mitochondria in muscle cells. Mitochondria are the cells' power plants, producing the energy required for everything the cell does. The study links the increase in mitochondria to improved aerobic capacity in the mice.

Although the research focused primarily on the effect of exercise on musclin levels, even when mice were sedentary, mice that lack musclin had decreased exercise endurance compared to sedentary wild-type mice, suggesting that musclin may promote muscle health even during the low level exercise of normal everyday living.

http://www.pnas.org/content/early/2015/12/09/1514250112