Histamine: Synthesis, Release And Functions
Histamine is an idazole amine that is involved in local responses of the immune system. This molecule also regulates normal functions in the stomach and acts as a neurotransmitter in the central nervous system (CNS). Outside of the CNS, it acts as a mediator of numerous physiological processes.
Since the 1950s, it has been known that this molecule is in the brain, but until recently its functions were unknown, which we will see later in more detail.
In the CNS, histamine is stored mainly in connective tissue mast cells and basophilic cells in the blood. These cells are eminently secretory and constitute a system that responds to a great variety of endogenous and exogenous stimuli through multiple cellular mechanisms.
Under normal conditions, mast cell histamine is stored in secretory granules that contain a matrix of heparin and various proteins. Along with various hydrolases, it is associated, for the most part, to the matrix by ionic bonds, but a small part may be in free form.
Release mechanisms
In order for histamine to be released, it must cross the granular and cell membranes. The release can be cytotoxic, after the rupture of both membranes, or exocytotic, through their previous fusion, without cellular damage.
Histamine is released in the course of physiological processes such as the secretion of gastric juice. However, its participation in pathological processes is much better known, in which it is released more or less explosively, such as in inflammatory reactions and immediate hypersensitivity reactions.
In these cases, histamine is often one of the mediators that are released simultaneously.
There are multiple physical and chemical agents that cause the release of histamine. Physicists include:
- Cold.
- Hot.
- Radiation
- Trauma
As for the seconds, the number and variety are extraordinary and they have to find receptor molecules in the membrane with which they interact.
Depending on the type of interaction, different sequences of steps will be triggered that will invariably end up raising the intracellular calcium concentration. Among these stimuli we can name, for example:
- Enzymes
- Antigens
- Cytotoxic agents.
General histamine response in the human body
When histamine is injected into the general circulation, it can cause flushing of the skin, tachycardia, throbbing headache, and hypotension. These effects are directly dependent on the dose, that is, by increasing the administered dose of histamine, the effects are potentiated.
Locally, it produces edema, itching, urticaria, and bronchoconstriction. Additionally, histamine contributes other mediators to anaphylactic shock. On the other hand, its role in the immune response, in inflammation, gastric secretion and circulatory regulation, among other functions that we will see below, is evident.
Functions of histamine in the body
There are many functions that histamine performs in the body. Among them we will highlight 5.
Allergic processes
Histamine is the most representative molecule in anaphylactic <em> shock </em>. In the face of allergens, there is a massive exit from the cells.
Histamine is considered a modulator of both the humoral and cellular immune responses, as well as the major mediator of hypersensitivity reactions. If applied in large doses or released during anaphylaxis, it causes a profound decrease in blood pressure.
Neurotransmission
On the other hand, histamine can act as a neuromodulator, regulating responses to other neutransmitters. Interacts like acetylcholine, opiates, GABA, etc. In addition, it increases the excitability of neurons in the CNS. Regulates hypothalamic functions, wake / sleep relationship, regulates appetite and vegetative functions.
Cardiovascular regulation
It acts as a vasodilator by interacting with its receptors. The release of histamine leads to an increase in capillary permeability due to effects on small vessels, as a consequence of the outflow of plasma proteins and fluids into the extracellular spaces.
Action on extravascular smooth muscle
This substance produces contraction of smooth muscles. Answers will vary widely even within the same person. Small doses of the molecule also trigger bronchoconstriction in people with bronchial asthma, as we saw earlier.
Gastric juice secretion
Histamine acts as a powerful gastric secretagogue and triggers the excretion of acid by the parietal cells. In addition, it also significantly increases the production of pepsinogen and intrinsic factor. Therefore, it is the predominant mediator of acid secretion from the stomach, by stimulation of H2 receptors.
