Our muscles have learned to keep us warm even when we are resting!

With low levels of mercury, mammals have an advantage over so-called cold-blooded creatures. Our muscles can act like furnaces, generating the heat needed to keep our body temperature stable.

But even when we are relaxed, our muscles can still generate heat, a technique called muscle thermogenesis.

As you sit and read this article, realize that the development of muscle-based thermogenesis was a key step in your evolution that allowed your ancestors to spread to less tropical places around the world.

Now, Australian researchers have identified a specific developmental pathway for mammalian muscle tissue from cold-blooded or “exothermic” animals.

“Cold-blooded animals like frogs and warm-blooded mammals like humans use the same basic muscle structures to create strength for posture and movement,” said Bradley Launikonis, a biomedical scientist at the University of Queensland.

But mammals achieved their geographic freedom by changing the way they regulate the concentration of calcium ions in their muscles at rest. This adaptation allows mammalian muscle cells to tolerate higher concentrations of calcium in the surrounding fluid, requiring the muscles to expend energy to excrete calcium.

Calcium ion pumps in skeletal muscle stabilize calcium ion levels. Previous research has shown that pumping activity also affects the amount of heat generated by skeletal muscles when they are at rest.

Even a small amount of heat generated builds up in each muscle fiber when you have enough skeletal muscle covering your body to allow your core temperature to remain constant in cold conditions. Add some insulation and that heat energy can make a big difference.

The researchers studied the muscle fibers of mammals and animals outdoors and compared how they function under the same conditions, and found that each of them counteracted the effects of increased concentrations of calcium ions differently.

They analyzed the dissolved calcium in the muscle fibers of frogs, mice, and humans with pernicious hyperthermia, a condition often caused by a mutation in the ryanodine receptor that makes calcium channels more likely to open when exposed to a stimulus.

Ryanodine receptors (RyR) are intracellular calcium channels in animal tissues such as muscle and nerve cells through which calcium ions pass. Calcium ion pumps work in the opposite direction, pumping calcium in the opposite direction, restoring homeostasis within cells.

A type called RyR1 is expressed in mammalian skeletal muscle, while exogenous animals express two types of receptors in their skeletal muscle, αRyR and βRyR.

The results of this study showed that in mammals, a sudden increase in calcium in the fluid surrounding relaxed muscle fibers causes ions to accumulate in a membrane-surrounded intracellular compartment called the sarcoplasmic reticulum rather than being rapidly released.

Normally, the influx of calcium ions into muscle cells stimulates RyR channels to release more calcium into the cytoplasm of the cell, which triggers a cascade leading to muscle contraction. However, it appears that mammals have developed some resistance to high levels of calcium in their muscle cells.

This is important because it ensures a constant leak of calcium ions from the sarcoplasmic reticulum, which causes the calcium ion pump to work harder, generating more heat.

The loss of one form of RyR appears to have helped mammalian muscles become less responsive to calcium ion stimuli, which in addition to metabolism maintains endotherm.

The study adds detail to our understanding of not only mammalian evolution but also our health, laying an important foundation for understanding how our muscles burn energy even when they are relaxed.

This study was published in The Proceedings of the National Academy of Sciences.

Source: Science Alert