ADRENALINE

So 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, narrows 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 "fight or flight" reaction.

Functions within the sympathetic nervous system

Together with norepinephrine, with which it shares origin and different actions, adrenaline is the typical "neurotransmitter" of the sympathetic nervous system. A violent and unexpected noise in the dark, the joking cry of a friend that suddenly appeared behind us or the screeching of chalk on the blackboard are examples of stressful situations that lead to massive activation of the sympathetic system.

In a few moments the heart increases the force and the contractile frequency, the bronchi, the pupil and the blood vessels of the appendicular muscles and the coronary system dilate, while glycogenolysis is stimulated in the liver. At the same time, again in order to prepare the body for the upcoming physical activity, the digestive processes are significantly slowed down, while the cutaneous and peripheral blood vessels constrict and blood pressure increases.

Many of the effects listed above 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 neurotransmitter dopamine, by β-oxidation to norepinephrine and finally by methylation to epinephrine.

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Secretion

The release of adrenaline is linked to the perception of stimuli such as 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; it is no coincidence that adrenaline is used in emergency therapy against anaphylactic shock, violent asthma attacks, symptomatic bradyarrhythmias and in cardiopulmonary resuscitation. Side effects include dyspnoea, vomiting, tachycardia, arrhythmia, anxiety, tremors, headaches and acute pulmonary edema, while the contraindications to the therapeutic use of adrenaline include diabetes, hypertension, hyperthyroidism, pregnancy and glaucoma.

Functions

Adrenaline produces a systemic effect by influencing the activity of almost all the tissues of the organism. To carry out its biological effects, adrenaline must interact with specific receptors, the so-called adrenergic receptors. There are essentially two types, α and Β, with various subtypes for each class; the different expression of these receptors and of the relative isoforms influences the various adrenergic activities at the tissue level. For example, we have seen that at the level of the bronchioles, adrenaline induces a dilation, while it has a constricting effect on the arteriolar smooth muscle.

Adrenaline increases glycogenolysis and hepatic and muscular gluconeogenesis, also stimulating lipolysis. It therefore facilitates the release of glucose and fatty acids, primary energy substrates to satisfy the body's energy requirements (increase in blood sugar and free fatty acids); as such it supports the metabolic activity of the organism. Among the other actions of adrenaline we remember: increased heart rate and respiratory rate, dilation of the pupils (important in situations where it is necessary to see in poor light), relaxation of the smooth muscles of the bronchioles (better supply of air to the alveoli pulmonary), increased blood pressure, vasoconstriction and selective vasodilation (reduces the blood supply to certain tissues, such as the skin, to increase it especially at the muscle level → relaxation of the smooth muscle of the arterioles 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 gastrointestinal secretion and motor activity, and of sexual arousal.

Norepinephrine, on the other hand, has a more specific activity at the circulatory level: it increases cardiac output, increases total peripheral resistance, therefore arterial pressure, and increases coronary flow.As shown in the figure, the stimuli necessary to produce consistent increases in the levels of norepinephrine in the blood have a much higher intensity and stress-generating power than those that raise adrenaline.