Histamine is a nitrogenous compound involved in digestive mechanisms, in the inflammatory response and as a neurotransmitter in various brain functions. In the human body, histamine is formed by decarboxylation of the amino acid L-histidine, through a reaction catalyzed by the enzyme histidine decarboxylase ; its degradation is instead entrusted to histaminase.
Although present in all tissues, histamine is produced, and mostly stored instantly, especially in mast cells and basophilic granulocytes (cells primarily involved in the allergic and immune response).
Histamine in brief Histamine is a nitrogenous substance involved in many cellular responses, such as inflammatory reactions and gastric secretion. Its massive release by cells involved in the inflammatory and immune response, determines:→ erythema, wheals (swelling), redness
→ increased mucus production in the airways (nose and bronchi)
→ onset of asthma symptoms
→ contraction of the muscles of the intestine (diarrhea and intestinal cramps).
Not surprisingly, the excessive release of histamine by these cells plays a leading pathophysiological role in mast cell-dependent inflammatory reactions and in IgE-mediated allergic diseases, such as asthma, urticaria, rhinitis and allergic conjunctivitis. the case that drugs particularly used to prevent these allergic manifestations are called antihistamines, as they are capable of counteracting the action of histamine at the receptor level.
In addition to the granules of basophils and mast cells, histamine is found in rather important concentrations also in the central nervous system and on the mucosa of the gastrointestinal tract.
Histamine receptors
Histamine carries out its action by binding to specific receptors located on the cell membrane, with different effects depending on the site and type of receptor with which it interacts. Currently, four types of histamine receptors are known, respectively defined as H1, H2 , H3 and H4.
• Smooth muscles (bronchi, intestines)
• Adrenal cortex
• Heart
• CNS
SMOOTH MUSCULATION OF THE BRONCHI: contraction of the bronchioles with the appearance of symptoms typical of asthma, decrease in lung capacity
SMOOTH MUSCULATURE OF THE INTESTINE: contraction that leads to intestinal cramps and diarrhea
INCREASE VASCULAR PERMEABILITY AND VASODILATION
MAINTENANCE OF THE WAKE STATE;
SENSORY FIBER STIMULATION: pain and itching
• Smooth vascular muscles
• Neutrophils • Heart • Uterus
VASODILATION: smooth muscle relaxation
INHIBITION OF LEUCOCYTARY FUNCTION
UTERINE CONTRACTION
• Enterochromaffin cells
centrally: histamine, acetylcholine, serotonin, dopamine;
peripherally: noradrenaline and acetylcholine, tachykinins.
GASTRIC SECRETION INHIBITION
• Eosinophils; Neutrophils
• Mononuclear, mast cells
Biological actions of histamine
Histamine is a substance with a vasodilatory, hypotensive and permeabilizing action, all very important characteristics in inflammatory phenomena; the slowing of blood flow and the increased permeability of the vessels in an area just hit by a trauma, in fact, allows the passage of white blood cells and other substances involved in confinement and damage repair. These actions give rise to the so-called "Triple Response of", which arises when histamine is injected transdermally:
- redness (by direct vasodilation);
- diffuse erythema (due to axon activation);
- wheal (due to increased permeability).
Look at random, just to remember how everything - when it comes to physiology - makes sense, mast cells are particularly abundant in the sites most exposed to potential tissue lesions (nose, mouth, feet, internal body surfaces, blood vessels, etc.) .
The plasma membrane of mast cells and basophils possess receptors for class E (IgE) immunoglobulins, typically involved in allergic reactions. Once these antibodies have been activated by a substance recognized as foreign, they bind to basophil and mast cell receptors, behaving in turn as real receptors. From this moment, at each subsequent contact with the antigen, the IgE will stimulate the degranulation of the basophils and mast cells to which they are bound, with consequent release of histamine and other substances involved in the allergic reaction.
At the level of the respiratory system, histamine once again causes dilation of the post-capillary venules and an increase in vascular permeability; it is also associated with a contraction of the bronchial smooth muscles and stimulates mucous secretion. In the presence of excessive bronchoconstriction, the calipers of the airways are reduced to the point of preventing normal oxygenation of the blood, with a sense of suffocation and hunger for air. During anaphylaxis, the enormous release of histamine and its bronchoconstrictor effect and local vasodilator, lead to the occlusion of the respiratory tract with serious danger for the patient's life.
At the gastric level, the enterochromaffin cells of the bottom of the stomach have the ability to release histamine, which acts in synergy with gastrin by stimulating the secretion of hydrochloric acid and intrinsic factor by the parietal cells, and of pepsin by the peptic cells.
At the enteric level, histamine causes contraction of the intestinal smooth muscles, triggering diarrhea at particularly high doses. This occurrence is typical of the consumption of foods rich in histamine (such as not very fresh fish) which cause redness of the face and neck, hives, nausea, vomiting, diarrhea, headache dizziness.
At the dermal level, histamine acts as a powerful stimulant of sensitive nerve endings, especially those that mediate pain and itching; this function is particularly evident following reactions from insect or nettle stings.
In the brain, the neurotransmitter histamine participates in various functions, such as neuroendocrine control, cardiovascular regulation, thermoregulation and wakefulness.
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