We discussed adrenaline in depth for your previous trait: Adrenaline: Baseline level.

To recap briefly, adrenaline is your ‘fight-or-flight’ hormone - it primes your body to deal with potential threats and stressors.

Whereas your previous trait looked at resting levels of adrenaline in the bloodstream, your latest Adrenaline: acute response trait analyzes your body’s ability to raise adrenaline levels in response to short-term physical and psychological stressors.

Exercise is one such physical stressor, and your blood levels of adrenaline rise acutely in response to physical activity of a sufficient intensity.

As we’ll explore in this article, the actions of adrenaline are beneficial during exercise, as they enhance the delivery of oxygen and nutrients to working muscles.

Adrenaline increases the supply of glucose for exercising muscles

One of the main fuel sources of exercising muscles is glucose.

During exercise, glucose undergoes a process of cell respiration, which yields a molecule called ATP. ATP is the chemical energy currency of our cells and is used to power muscle contraction.

To ensure we have plenty of fuel reserves, our muscles and liver join glucose molecules together into long chains to form glycogen. Glycogen is one of our body’s energy storage molecules. When our body has increased energy needs, during exercise for example, we can quickly break down glycogen to provide us with a fresh supply of glucose. Glucose can then be used generate more ATP to power further muscle contraction.

So how does adrenaline fit into this picture?

By binding to α1 and β2 adrenergic receptors in liver and muscle tissue respectively, adrenaline enhances the breakdown of glycogen to produce glucose. We call this process glycogenolysis.

In addition to stimulating glycogenolysis, adrenaline also inhibits the action of insulin. As you may recall from your Insulin and blood glucose levels trait, insulin is the hormone that facilitates the uptake of glucose from the bloodstream into cells.

By suppressing this action of insulin, adrenaline ensures levels of glucose in the blood are kept high, providing a steady fuel source for exercising muscles.

For similar reasons, adrenaline also inhibits the uptake and conversion of glucose into glycogen by the liver. This process is called glycogenesis.

KEY POINTS

• Glucose is a key fuel source for exercising muscles.
• Adrenaline enhances the breakdown of glycogen in the liver and skeletal muscles to yield glucose.
• Adrenaline inhibits the conversion of glucose into glycogen by the liver.
• Adrenaline inhibits the action of insulin on blood glucose levels.

Adrenaline breaks down fat for energy during exercise

Glucose isn’t the only energy source for our exercising muscles. We can also burn fat for fuel.

In fact, during mild-intensity exercise, our skeletal muscles predominantly use fatty acids (one of the building blocks of fats) as an energy source.

Adrenaline aids this process by ensuring our muscles have a steady supply of fatty acids provided by our stored fat (adipose) tissue.

Our fat cells (adipocytes) in adipose tissue express β1 adrenergic receptors on their surface membrane. When adrenaline binds to and activates these receptors, it stimulates an enzyme called adipose triglyceride lipase. This enzyme breaks down our fat stores (in the form of triglycerides) into fatty acids and glycerol. Fatty acids can then be used for energy (through a process called fatty acid- or beta- oxidation) by our working muscles.

Furthermore, adrenaline also causes widening (vasodilatation) of the blood vessels supplying our adipose tissue. This means fatty acids are more quickly released into the blood stream, where they can subsequently taken up and used for energy by exercising muscles.

Interestingly, the more you exercise, the more efficient your body becomes at breaking down fat. Studies show that endurance training causes adipose tissue to become more sensitive to the fat-burning effects of adrenaline.

KEY POINTS

• Adrenaline enhances the breakdown of fat into fatty acids.
• Our muscles use fatty acids as a source of energy, particularly during low-intensity, long-duration exercise.

Adrenaline increases heart rate

Here’s a simple experiment you can perform at home.

First, feel your pulse by placing your index and middle finger on your radial artery, which runs along the outer part of your wrist.

Now jog on the spot at high intensity for 30 seconds.

After 30 seconds, immediately re-take your pulse. Have you noticed anything?

Your pulse rate ought to have increased considerably.

This is due to the effect of adrenaline on the sinoatrial node: your heart’s natural pacemaker. The sinoatrial (SA) node sends out electrical impulses at regular intervals, which then are conducted to heart muscle cells, causing them to contract in a rhythmical fashion.

When adrenaline binds to β1 adrenergic receptors on your SA node, it activates specialized ion channels, which allow calcium ions to enter. This changes the electrical properties of the SA node, causing an increase in the rate at which it sends out electrical impulses. The end result is that your heart beats faster.

By speeding up your heart rate, adrenaline ensures oxygen and nutrients in the bloodstream are delivered to your exercising muscles more quickly.

KEY POINTS

• Adrenaline raises heart rate by acting on the sinoatrial node - your heart's natural pacemaker.

Adrenaline increases the force of heart muscle contraction

In addition to speeding up your heart rate, adrenaline increases the force at which the heart contracts. In scientific terms, we say that there is an increase in the contractility of the heart muscle.

As a result of higher contractility, the heart can expel blood more forcefully, allowing a greater amount of blood to be delivered to working muscles with each heartbeat.

Similar to its effect on the heart’s pacemaker (sino-atrial node), adrenaline increases the contractility of heart muscle by binding to β1 adrenergic receptors. This leads to an increase of calcium ions within cardiac muscle cells (cardiac myocytes), which enhances a process called excitation-contraction coupling. Put simply, excitation-contraction coupling refers to the biochemical processes within a cell that allow it to convert an electrical impulse into mechanical contraction of muscle fibers.

KEY POINTS

• Adrenaline increases the force of contraction by heart muscle cells

Adrenaline increases the force of skeletal muscle contraction

It isn’t just heart muscle contraction that is enhanced by adrenaline. Many people report increased skeletal muscle strength during an ‘adrenaline rush’ – for example, in an intense sports competition, fight, or emergency situation.

We describe adrenaline as having a positive inotropic effect on muscle – it increases the force of muscle contraction.

This effect is likely to be caused by adrenaline binding to β2 adrenergic receptors expressed by skeletal muscle cells. Studies suggest that, by binding to β2 receptors, adrenaline increases the concentration of calcium ions within muscle cells, which gives rise to more forceful muscle contraction. It is currently a subject of debate of just how high your blood adrenaline levels need to be to elicit this increase in muscular strength.

KEY POINTS

• Adrenaline increases the force generated by skeletal muscle.

Adrenaline dilates blood vessels supplying skeletal muscle

As explained in your Nitric oxide and blood flow trait, exercising muscles have higher energy demands and therefore require a greater supply of glucose, as well as oxygen, to yield energy from glucose. Muscle contraction also produces metabolites, which need to be cleared away.

In order to meet these needs, we increase blood flow to exercising muscles. Adrenaline aids this process.

When adrenaline binds to β2 adrenergic receptors on vascular smooth muscle cells within arterial walls, it causes them relax. This widens the arteries (a process called vasodilatation), allowing more blood to flow to muscles.

To complement this, adrenaline also helps to constrict (narrow) arteries supplying organs that are less essential during exercise (e.g. the gut, kidneys and skin ).

KEY POINTS

• Adrenaline causes vasodilatation (widening) of arteries supplying exercising muscles.
• Adrenaline causes vasoconstriction (narrowing) of arteries supplying the gut, kidneys and skin.

Adrenaline dilates airways

Our exercising muscles require oxygen to yield energy from glucose (via aerobic respiration) and fatty acids (via beta-oxidation).

To facilitate the delivery of oxygen to muscles, adrenaline widens our airways (bronchi and bronchioles) in our lungs. Just as we have smooth muscle in our blood vessel walls, we also have rings of smooth muscle that surround our airways (bronchiolar muscle in the above diagram).

When adrenaline binds to β2 adrenergic receptors on airway smooth muscle cells, it causes them to relax. This dilates our airways (a process called bronchodilation), allowing more air to flow into our air sacs (alveoli) and oxygen diffuse into our bloodstream.

Dilation of our airways also allows more carbon dioxide, which is a waste production of cell respiration, to be breathed out.

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KEY POINTS

• Adrenaline dilates our airways allowing more oxygen to be delivered to exercising muscles.

Genetics

As with your Adrenaline: Baseline Level trait, genes that encode the enzymes which synthesize and clear adrenaline will influence your levels of adrenaline during exercise.

On this note, our TrueTrait ™ algorithm factors in variants of your MAO-A, COMT and PNMT genes.

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