Catecholamines, which include dopamine, norepinephrine and epinephrine, are hormones and neurotransmitters that play an important role in the sympathetic nervous system. They are released during times of stress or danger and produce the “fight or flight” response. One of the main effects of catecholamines is to increase metabolism, which provides the body with extra energy to respond to threats.
What are catecholamines?
Catecholamines are derived from the amino acid tyrosine. The main catecholamines are:
- Dopamine
- Norepinephrine
- Epinephrine (adrenaline)
Dopamine is produced in the brain and functions as a neurotransmitter. Norepinephrine and epinephrine are produced in the adrenal glands and function as hormones. When the sympathetic nervous system is activated, the adrenal glands release norepinephrine and epinephrine into the bloodstream.
How do catecholamines increase metabolism?
Catecholamines increase metabolism in a number of ways:
- Increase breakdown of glycogen into glucose – Catecholamines stimulate the breakdown of glycogen, stored in the liver and muscles, into glucose. This provides extra fuel for energy production.
- Increase lipolysis – Catecholamines stimulate fat cells to break down triglycerides into fatty acids and glycerol. This provides free fatty acids that can be used for energy production.
- Increase heart rate and contractility – By increasing heart rate and contraction force, more oxygenated blood is pumped through the body to supply oxygen and nutrients to tissues.
- Dilate bronchioles – Catecholamines relax the airways, allowing more air to get into and out of the lungs. This improves oxygen exchange.
- Increase blood flow to muscles – Catecholamines redirect blood away from the skin and viscera toward skeletal muscle beds. This allows more delivery of oxygen and energy substrates.
- Increase cellular glucose uptake – Catecholamines increase the amount of glucose transported into cells via GLUT4 glucose transporters, providing more substrate for energy production.
Together, these effects allow cells to produce more ATP energy rapidly through aerobic respiration and anaerobic glycolysis pathways.
Metabolic effects of epinephrine vs norepinephrine
Although epinephrine and norepinephrine both increase metabolism overall, studies show they have some differing metabolic effects:
- Epinephrine is more potent at stimulating glycogen breakdown.
- Norepinephrine produces greater increases in lipolysis.
- Norepinephrine causes greater vasoconstriction peripherally, while epinephrine increases blood flow.
- Epinephrine increases hepatic glucose production more than norepinephrine.
So while both catecholamines mobilize energy stores, epinephrine acts faster to raise blood sugar, while norepinephrine has greater longer-lasting lipolytic effects.
Do catecholamines increase resting metabolic rate?
Resting metabolic rate (RMR) refers to the number of calories burned at rest by vital body functions. Studies show that catecholamines can increase RMR:
- Infusion of epinephrine was shown to increase RMR by up to 29% in healthy men.
- Men given a norepinephrine reuptake inhibitor had a 9% increase in RMR.
- Higher norepinephrine levels were associated with greater RMR in a study of healthy women.
The increases in RMR are likely due to catecholamines stimulating increased Na/K ATPase activity. This sodium-potassium pump is responsible for a major portion of ATP usage at rest.
Do catecholamines affect metabolism differently in obesity?
Obesity is characterized by catecholamine resistance, where fat and muscle cells have a blunted metabolic response to catecholamines. Reasons for this include:
- Downregulation of beta-adrenergic receptors
- Impaired adenylate cyclase activity
- Lower lipolytic response to norepinephrine
This catecholamine resistance means the metabolic rate increase in response to catecholamines is blunted in those who are obese. Weight loss can help restore catecholamine responsiveness.
Conclusion
In summary, catecholamines like epinephrine and norepinephrine play an important role in rapidly increasing energy mobilization from glucose and fat stores during stress. They increase metabolism by stimulating glycogenolysis, lipolysis, thermogenesis and oxygen delivery to tissues. Catecholamine resistance in obesity can impair these metabolic effects. Appropriate catecholamine signaling is therefore important for maintaining a responsive metabolic rate.