What is adrenaline and what does it do?
Adrenaline (also known as epinephrine) is a hormone that primarily influences the body's fight or flight response. It is produces by adrenal glands, which are glands located on top of the kidneys. Additionally, some neurons (cells of the nervous systems) also produce adrenaline. The main function of adrenaline addresses every organism's main task: survival.
When the body perceives a threat, adrenaline is released, which in turn increases heart rate and blood flow to the brain and muscles, as well as changes the body's metabolism (how we digest food and produce, store and release energy) [1].
Adrenaline influences several pathways and systems in the body:
- Metabolic pathways: Adrenaline helps mobilise energy stores to prepare the body for physical activity. This is achieved by suppressing secretion of insulin (hormone that causes glucose to be stored inside cells) and upregulation of release of another hormone, glucagon, from pancreatic cells. Glucagon acts to release glucose from cells into the bloodstream, so that it can be delivered to cells that need energy [2]. Additionally, adrenaline signals to induce production of more adrenaline.
- Cardiovascular system: Adrenaline acts to increase heart rate and blood pressure in order to increase perfusion of tissues (how much blood they are getting). This is achieved via binding of adrenaline to beta-adrenergic receptors which are present in the heart. This in turn results in accelerating the speed of ventricular contraction and increase of the volume of blood ejected by the left ventricle (which pumps oxygenated blood) [3].
- Immune system: Adrenaline likely interacts with the immune system via increasing the release of interleukin-6, which is a molecule that signals to the immune system to respond with an inflammatory reaction [4]. Adrenaline also changes how cells of the immune system, predominantly macrophages, metabolise glucose and other energy molecules [5].
Do animals have adrenaline?
As adrenaline is very important to muscle functions and response to potentially life-threatening situations, it is very abundant across nature. Therefore, the simple answer is yes, animals do have adrenaline. Furthermore, given its importance, adrenaline is evolutionarily conserved, meaning that the human adrenaline molecule looks and acts the same than its animal counterparts.
However, while the molecule looks the same, there are still differences between how human and animal bodies handle and utilise adrenaline:
- Receptors: Adrenaline binds to adrenergic receptors in multiple tissues. The number (abundance) and affinity (how easily they bind the adrenaline molecule) varies between species. For example, blood platelets bind adrenaline, however humans, dogs, and rabbits have a high affinity for adrenaline, whereas rat and guinea pig platelets do not respond to adrenaline in the same way [6]. Additionally, the distribution of receptors and their subtypes varies across species as well. Rat cortex and lung possess both beta 1 and beta 2 receptor subtypes, while rat erythrocytes only have beta 2 sites, whereas human erythrocytes also express the alpha type of receptor [7].
- Concentrations: The concentration of adrenaline in the body can vary between species (and tissues). For example, frogs have a very high concentration of adrenaline in the hypothalamus, whereas rabbits, horses, and guinea pigs have almost undetectable levels [8].
What is adrenaline used for?
Given its functions to increase heart rate and blood pressure, adrenaline is a useful molecule in the medical field [2]. It is routinely used as a drug across multiple fields of medicine, and helps save countless lives daily.
Perhaps the most prominent medical use of adrenaline is the treatment of anaphylaxis - a type of very severe allergic reaction which can be fatal if untreated. An injection od adrenaline helps open up the airway that closes up in the anaphylactic reaction, allowing the person to breathe. Adrenaline (epinephrine) is the medication that is inside the Epi pen [9].
Similarly, adrenaline is also used in treatment of cardiac arrest - adrenaline injection is used in the cardiopulmonary resuscitation pathway since 1896. The rationale behind adrenaline usage is that it diverts bloodflow from the peripheral tissues to protect essential organs like the heart or the brain [10]. This protects the tissues from ischemic damage and increases the likelihood of return of spontaneous circulation.
Given the effects on circulation and blood pressure, adrenaline is also used in treatment of severe infections such as sepsis or septic shock, where the patient's blood pressure needs to be increased to maintain physiological function [9]. Here, adrenaline is given as an intravenous infusion.
Furthermore, adrenaline is used in the formulation of local anaesthetics - adding adrenaline to the injection can make the anaesthetic effect last longer and also minimise bleeding [9].
Additionally, inhaled form of adrenaline is used to treat asthma attacks [9].