Exercise, Muscle as an Endodcrine Organ, Glucose Metabolism

Exercise, Muscle as an Endodcrine Organ, Glucose Metabolism

Muscle Hormones and Enzymes secreted during and after Exercise

Exercise triggers a complex cascade of chemical signals designed to manage energy, repair tissue, and improve mood. These substances are categorized into hormones (systemic messengers), myokines (muscle-derived signaling proteins), and metabolic enzymes.

 

1. Hormones Secreted During & After Exercise 

Hormones are released into the bloodstream to coordinate the body’s response to the stress of physical activity. 

• Catecholamines (Adrenaline & Noradrenaline): Secreted by the adrenal glands during exercise to increase heart rate, boost blood pressure, and break down glycogen for immediate energy.
• Cortisol: A “stress hormone” from the adrenal glands that rises during intense or prolonged activity to mobilize fat and protein for fuel.
• Growth Hormone (GH): Released by the pituitary gland during and shortly after exercise (peaking within 15–30 minutes) to stimulate tissue repair, bone growth, and fat metabolism.
• Testosterone: Levels increase temporarily during and for about 30–45 minutes after high-intensity resistance training to support muscle protein synthesis and repair.
• Glucagon & Insulin: The pancreas increases glucagon to raise blood sugar while suppressing insulin to prioritize glucose delivery to working muscles.
• Endorphins: Released by the pituitary gland and hypothalamus during exercise to act as natural pain relievers and mood enhancers. 

 

2. Myokines (Muscle-Derived Signaling Proteins)

Skeletal muscle acts as an endocrine organ, secreting “myokines” directly from contracting fibers that communicate with other organs. 

• Interleukin-6 (IL-6): The most studied myokine; its levels can increase up to 100-fold during exercise. It helps regulate glucose uptake, fat oxidation, and has anti-inflammatory effects.
• Irisin: Released during strength training and HIIT; it promotes “browning” of white fat cells, which increases energy expenditure and improves insulin sensitivity.
• BDNF (Brain-Derived Neurotrophic Factor): While also in the brain, it is produced in muscles during exercise and is linked to improved memory and learning.
• Myostatin: Exercise, particularly resistance training, actually decreases the secretion of this protein, which normally acts to inhibit muscle growth. 

 

3. Key Muscle Enzymes

Enzymes catalyze the chemical reactions necessary for muscle contraction and energy production. 

• Creatine Kinase (CK): A key enzyme for energy (ATP) buffering; its presence in the blood after exercise is a primary biomarker of muscle damage and the recovery process.
• Glycogen Phosphorylase: Activated during exercise to break down stored muscle glycogen into glucose for fuel.
• Pyruvate Dehydrogenase (PDH): The rate-limiting enzyme for carbohydrate oxidation, which transitions to an active form during exercise to help produce energy in the mitochondria.
• Antioxidant Enzymes (SOD, Catalase): Exercise training increases levels of Superoxide Dismutase (SOD) and Catalase to neutralize reactive oxygen species (ROS) produced during intense activity.
• AMPK (AMP-activated protein kinase): Often called a “metabolic master switch,” it is activated by the energy drain of contraction to stimulate glucose uptake and fat burning. 

 

Reference:

This excerpt from Strength Training, Second Edition briefly details hormones and their role in the human body.

The following is an exclusive excerpt from the book Strength Training, Second Edition, published by Human Kinetics. All text and images provided by Human Kinetics.

Nearly every physiological function in the body is regulated by hormones—chemical messengers that travel via the bloodstream to target tissues in the body (e.g., muscle). Figure 4.1 provides an overview of the roles that hormones play. Because resistance exercise dramatically affects hormonal responses in the body and in part stimulates the development of tissues (including muscle and bone), you need to understand how the food you eat affects these hormones. Hormones play a significant role in metabolic balance. They largely are responsible for the fuel selection, partitioning of nutrients, and gene regulation that ultimately affect body composition and muscle mass.

 

The quantity, quality, and timing of nutrient consumption in resistance training may have a large influence on the hormones that regulate fuel selection during training, glycogen repletion and protein synthesis. Proper consumption of nutrients can result in optimal gains in muscle strength and power.

Acute bouts of resistance exercise (considering the load, number of sets, number of repetitions, and number and length of rest intervals) create a stimulus that generates a hormonal response. Muscle actions trigger a series of mechanical and chemical events in the muscle that signal hormones to regulate enzymes, which in turn regulate the genetic formation of proteins. For anabolic actions, just the muscle fibers that are activated as part of the motor unit used to perform the exercise will be affected. Conversely, catabolic factors (e.g., reactive oxygen species, cortisol, and so on) can target all muscle fibers. Some nutrients, such as the branched-chain amino acid leucine, can stimulate protein synthesis directly in muscle. Exercise increases blood flow and thus increases the delivery of hormones and nutrients to the target receptors on and in the muscle cells. The anabolic environment is enhanced by the exercise stimuli along with the availability of nutrients and hormones. Consumption of carbohydrate and protein appears to affect the responses of hormones involved in muscle metabolism—including insulin, testosterone, growth hormone, cortisol, estrogen, and insulin-like growth factor-1—thus affecting muscle protein and glycogen balance. Although the role of insulin in response to exercise and diet is understood, the influence of the other hormones listed remains unclear. Metabolic context is the important factor related to the role of certain hormones and their effects.

Resistance exercise stimulates the anterior pituitary gland to secrete GH in its various forms arising from two types of somatotrophs called band 1 and band 2 granules. Band 1 cells secrete the familiar 191-amino acid form of GH and smaller molecular weight peptides, and band 2 cells secrete larger bioactive aggregate forms of GH. The larger bioactive aggregates make up more than 20-40 times the amount of GH in the blood and most likely are responsible for many of the functions of GH, thus a super family of biologically active peptides; (PL = prolactin).

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