Skeletal muscle is now recognized as a sophisticated endocrine organ. This shift is driven by the discovery of myokines, small proteins and peptides released by muscle cells in response to contraction. These molecules act as chemical messengers, traveling through the bloodstream to communicate with distant tissues. Myokines are a key mechanism explaining how physical activity promotes health across multiple organ systems, including the brain and various metabolic organs.
Defining Muscle as an Endocrine Organ
The concept of muscle functioning as an endocrine organ emerged with the identification of Interleukin-6 (IL-6) being produced and released by skeletal muscle fibers following exercise. Myokines are released specifically when muscles contract, making their secretion a direct, activity-dependent response.
Once secreted, these messengers exert their effects in three main ways: autocrine (acting on the muscle cell that released them), paracrine (acting on nearby cells), and endocrine (acting on distant organs via the bloodstream). This endocrine function allows muscle to participate actively in whole-body communication. Besides IL-6, other foundational myokines include Irisin and Myostatin, though hundreds of these signaling molecules have been identified.
Myokines and Systemic Metabolic Regulation
Myokines play a role in maintaining metabolic balance throughout the body, influencing organs responsible for energy storage and processing. Their actions address public health concerns like obesity and insulin resistance.
In adipose tissue, myokines like Irisin and IL-6 promote the “browning” of white fat. While white adipose tissue stores energy, browning converts these cells into beige fat cells, which increase energy expenditure through thermogenesis. This mechanism links muscle activity to improved body composition.
Myokines also significantly impact the liver and overall glucose homeostasis. Interleukin-6 stimulates glucose production in the liver during exercise to fuel the working muscles. Simultaneously, myokines enhance insulin sensitivity in muscle and other tissues, improving the body’s ability to utilize glucose efficiently and regulate blood sugar levels. Irisin has also been shown to enhance insulin secretion from pancreatic beta cells.
Muscle signaling extends to skeletal health. Muscle contraction releases signals that support bone density and strength. This muscle-bone cross-talk helps ensure the skeletal system can withstand the forces generated by active musculature.
Myokine Influence on Brain Function
The muscle-brain axis describes the communication pathway mediated by myokines that links skeletal muscle activity directly to central nervous system function. Myokines are capable of crossing the blood-brain barrier, allowing them to exert direct effects on brain cells.
Once inside the brain, myokines facilitate neurogenesis, particularly in the hippocampus, a region important for memory and learning. They also promote synaptic plasticity. This enhanced plasticity is fundamental for cognitive function and memory consolidation.
A primary mechanism involves myokines like Irisin and Cathepsin B (CTSB) stimulating the expression of brain-derived neurotrophic factor (BDNF). BDNF supports the survival of existing neurons and encourages the growth and differentiation of new ones. By increasing BDNF levels, myokines help regulate mood and cognitive performance while offering protection against neurodegenerative processes.
Stimulating Myokine Production Through Exercise
The release of myokines is directly proportional to the intensity and duration of muscle contraction, making exercise the most effective way to harness their benefits. High-intensity exercise, including both resistance training and high-intensity interval training (HIIT), generally produces a stronger burst of myokines compared to low-intensity steady-state activity. For instance, levels of IL-6 can increase several-fold following a bout of intense endurance exercise.
Resistance training is particularly potent for stimulating myokine release, increasing levels of myokines like IL-6 and IL-15. The type of muscle contraction is also significant. Eccentric contractions, where the muscle lengthens under tension (such as lowering a weight), can induce a greater myokine response than concentric contractions (lifting a weight).
To maintain elevated myokine levels and their systemic health advantages, exercise frequency is important. A single bout of exercise causes a transient spike, with myokine levels typically returning to baseline within a few hours. Consistent, regular physical activity is necessary to ensure a sustained, positive signaling environment across the body’s organ systems. By strategically integrating varied intensities and contraction types, individuals can maximize the muscle’s role as a communicator for metabolic and neurological health.