What is adrenaline
Adrenaline, or epinephrine, is a hormone synthesized in the internal (medullary) portion of the adrenal gland. Once secreted and released into the circulation, adrenaline accelerates the heart rate, restricts the caliber of blood vessels, dilates the bronchial airways and enhances physical performance; substantially, therefore, adrenaline improves the reactivity of the organism, preparing it in a very short time for the so-called "attack or flight" reaction.
Functions within the sympathetic nervous system
Together with norepinephrine, with which it shares origin and various actions, adrenaline is the "neurotransmitter" typical of the sympathetic nervous system. A violent and unexpected noise in the dark, the playful scream of a friend who suddenly appeared behind us or the screeching of a chalk on the blackboard, are examples of stressful situations that lead to a massive activation of the sympathetic system.
In a few moments the heart increases the strength and the contractile frequency, the bronchi, the pupil and the blood vessels of the appendicular and coronary system dilate, while at the liver level the glycogenolysis is stimulated. At the same time, again in order to prepare the body for impending physical activity, the digestive processes are significantly slowed down, while the cutaneous and peripheral blood vessels become constricted and the arterial pressure increases.
Many of the effects just listed are mediated by adrenaline, which as mentioned is synthesized at the adrenal level starting from tyrosine: the first reaction is the oxidation of the amino acid into dihydroxyphenylalanine (L-DOPA), followed by decarboxylation in the dopamine neurotransmitter, from β-oxidation in noradrenaline and finally by methylation in epinephrine.
Due to its chemical structure in which, similarly to noradrenaline, an amino group and an ortho -dihydroxy-benzene called catechol are recognized, adrenaline belongs to the catecholamine class.
Secretion
The release of adrenaline is linked to the perception of stimuli as a physical threat and fear, excitement, loud noises, intense light and high ambient temperature; all these stimuli are processed at the hypothalamic level, where they evoke a response of the parasympathetic nervous system.
Other stimuli are represented by hypovolemia, hypoxia, hypotension, hypoglycemia, pain and stress; not by chance the adrenaline is used in emergency therapy against anaphylactic shock, violent asthmatic attacks, symptomatic bradyarrhythmias and in cardiopulmonary resuscitation. Side effects include dyspnea, vomiting, tachycardia, arrhythmia, anxiety, tremors, headache and acute pulmonary edema, while contraindications to the therapeutic use of adrenaline include diabetes, hypertension, hyperthyroidism, pregnancy and glaucoma.
Functions
Adrenaline produces a systemic effect influencing the activity of almost all the tissues of the body. To perform its biological effects, adrenaline must interact with specific receptors, the so-called adrenergic receptors. Two types are essentially recognized, α and Β, with various subtypes for each class; the different expression of these receptors and their isoforms influences the various adrenergic activities at tissue level. For example, we have seen that at the level of the bronchioles adrenaline induces dilation, while it has a constrictive effect on arteriolar smooth muscle.
Adrenaline increases glycogenolysis and hepatic and muscle gluconeogenesis, also stimulating lipolysis. It therefore facilitates the release of glucose and fatty acids, primary energy substrates to meet the body's energy demands (increase in blood sugar and free fatty acids); as such it supports the metabolic activity of the organism. Other adrenaline actions include: increased heart rate and respiratory rate, pupil dilation (important in situations where it is necessary to see in low light), relaxation of the smooth muscles of the bronchiole walls (better supply of air to the alveoli pulmonary), increased blood pressure, vasoconstriction and selective vasodilatation (reduces blood supply to certain tissues, such as the skin, to increase it above all in the muscle level → relaxation of the smooth muscle of the arteries of the skeletal muscles and favorable effects on muscle contraction → fatigue later). At the same time there is an inhibitory activity of some "non-essential" processes: for example, inhibition of secretion and gastrointestinal motor activity, and sexual excitement.
receptors
Notes on adrenaline and noradrenaline receptors
| Recet. | Sensitivity | Headquarters | Main actions of the agonists |
| α1 | Noradrenaline> adrenaline | Most target tissues | Smooth muscle contraction, with hypertensive effect |
| α2 | Adrenaline ≥ Noradrenaline | Gastrointestinal tract and pancreas | Smooth muscle contraction, decreased insulin secretion and increased glucagon, inhibited release of neurotransmitters (decreased production of noradrenaline and acetylcholine), contraction of the sphincters of the gastrointestinal tract |
| β1 | Adrenaline = Noradrenaline | Cardiac muscle, kidney | Increased contraction and heart rate, increased renin release, lipolysis stimulation in adipose tissue |
| β2 | Adrenaline > Noradrenaline | Some blood vessels and smooth muscle of some organs (bronchial smooth muscle, gastrointestinal, coronary) and large vessels supplying the skeletal musculature | Vasodilation, stimulation of lipolysis, anabolism and vasodilation in the muscle, with increased physical performance, stimulation of glycolysis and gluconeogenesis, increased renin secretion, sphincter contraction of the gastrointestinal tract |
| β3 | Norepinephrine > adrenaline | Adipose tissue | Stimulus on lipolysis |
