Target cells are such structural and functional units that specifically interact with hormones using special receptor proteins. The definition is generally clear, but the topic itself is very voluminous, and each of its aspects is certainly important. It is quite difficult to cover all the material at once, so now we will only talk about the main points regarding target cells, their types and mechanism of action.
Definition
Target cells is a very interesting term. The prefix present in it is logically justified. After all, in fact, every cell in the body is a target for hormones. At the moment of their contact, a specific biochemical reaction is launched. The process carried out next is directly related to metabolism.
How strongly the effect will be realized determines the concentration of the hormone that reacted with the target cell. This, however, is not the only key factor. Also plays a rolethe rate of hormone biosynthesis, the conditions for its maturation, and the specifics of the environment in which the cell contacts with the carrier protein.
In addition, the biochemical effect reflects the antagonism or synergism of hormonal effects. For example, epinephrine and glucagon (produced in the adrenal glands and pancreas, respectively) have a similar effect. Both hormones activate the breakdown of glycogen in the liver.
But the female sex hormones progesterone and estrogen have an antagonistic effect. The first slows down the contraction of the uterus, and the second, on the contrary, strengthens them.
The concept of receptor proteins
It should be studied in a little more detail. Target cells are, as already mentioned, structural units that interact with hormones. But what are the notorious receptor proteins? So called molecules that have two main functions:
- React to physical factors (light, for example).
- Bind other molecules that carry regulatory signals (neurotransmitters, hormones, etc.).
The last feature is the most significant. Due to the conformational changes that these signals induce, receptor proteins trigger certain biochemical processes in the cell. The result is the realization of her physiological response to external signals.
Proteins, by the way, can be located on the nuclear or outer membrane of the cell or in the cytoplasm.
Receptors
About themmust be told separately. Target cell receptors are their specific chemical structures that contain complementary sites that bind to a hormone. It is as a result of this interaction that all subsequent biochemical reactions occur, which lead to the final effect.
It is important to note that the receptor of any hormone is a protein with at least two domains (tertiary structure elements) that differ in structure and function.
What are their functions? The receptors work as follows: one of the domains binds the hormone, and the second produces a signal that is applicable to a specific intracellular process.
In steroid biologically active substances, everything happens a little differently. Yes, hormone receptors of this group also have at least two domains. Only one of them carries out binding, and the second is associated with a specific DNA region.
It is interesting that in many cells there are so-called reserve receptors - those that are not involved in the formation of a biological response.
Important to know
Studying the pathways of action of hormones on target cells and other features of this topic, it must be noted that so far most of the receptors have not been sufficiently studied. Why? Because their isolation and further purification are difficult. But the content in the cells of each receptor is quite low.
However, it is known that hormones interact with receptors in a chemical-physical way. Hydrophobic andelectrostatic connections. When the receptor binds to a hormone, the receptor protein undergoes a conformational change, resulting in its activation with the signaling molecule complex.
Neurotransmitters
This is the name of biologically active substances, the main function of which is to transmit electrochemical impulses from nerve cells and neurons. They are also called "intermediaries". Of course, target cells are also affected by neurotransmitters.
More precisely, the “intermediaries” contact directly with the biochemical receptors mentioned above. The complex that these two substances form is able to influence the intensity of certain metabolic processes (through the target of mediators or directly).
For example, one neurotransmitter causes an increase in the excitability of the target cell and a gradual depolarization of the postsynaptic membrane. Other "intermediaries" can have a completely opposite effect (inhibitory).
Another number of substances block and activate receptors. These include prostaglandins, neuroactive peptides and amino acids. But in fact, there are many more substances that affect the process of information transfer.
Types of action of hormones on target cells
There are five in total. You can select these species in the following list:
- Metabolic. Manifested in a change in the permeability of cell membranes, organelles, as well as the activity of intracellular enzymes and their synthesis. Pronounced metabolic effectdifferent hormones produced by the thyroid gland.
- Corrective. This action affects the intensity of the functions provided by the target cells. Its severity depends on the reactivity and the initial state. As an example, we can recall the effect of adrenaline on heart rate.
- Kinetic. When such an impact is exerted, target cells move from a calm state to an active one. A striking example is the reaction of the muscles of the uterus to oxytocin.
- Morphogenetic. It consists in changing the size and shape of target cells. Somatotropin, for example, affects the growth of the body. And sex hormones are directly involved in the formation of sexual characteristics.
- Reactogenic. As a result of this action, the sensitivity of target cells, their susceptibility to other mediators and hormones changes. Cholecystokinin and gastrin affect the excitability of nerve cells.
Interaction with water-soluble hormones
He has his own specifics. Talking about the interaction of hormones with target cells, it must be noted that if they are water-soluble, then they have an effect without penetrating inside - that is, from the surface of the cytoplasmic membrane.
Here are the steps involved in this process:
- Formation on the membrane surface of the HRK (hormone-receptor complex).
- Subsequent enzyme activation.
- Formation of secondary intermediaries.
- Formation of protein kinases of a certain group (enzymes that modify other proteins).
- Activation of protein phosphorylation.
The described process, by the way, is properly called reception.
Interaction with fat-soluble hormones
Or, as they are most often called, with steroids. In this case, there is a different effect of hormones on target cells. Because steroids, unlike water-soluble substances, still penetrate inside.
Step by step it looks like this:
- The steroid hormone contacts the membrane receptor, after which the GRK is transferred into the cell.
- The substance then binds to the cytoplasmic receptor protein.
- After that, the GRK is transferred to the core.
- Interaction with the third receptor is carried out, which is accompanied by the formation of GRK.
- GRK then binds to DNA and, of course, to the chromatin acceptor.
By studying this pathway of hormone action on target cells, one can understand that GRK has been present in the nucleus for quite a long time. Therefore, all physiological effects occur several hours after the start of the process.
Signal recognition
And a few words about this are also worth saying. Signals entering the body are of two types:
- External. What does it mean? The fact that signals to the cell come from the external environment.
- Domestic. Signals are formed and then act in the same cell. Often the signals are metabolites that play the role of allosteric inhibitors or activators.
Regardless of the type, they have the same tasks. They can be identified insuch a list:
- Exclusion of so-called idle metabolic cycles.
- Maintaining the proper level of homeostasis.
- Intercellular and internal coordination of metabolic processes.
- Regulation of the processes of formation and further use of energy.
- Adaptation of the body to changes in the environment.
In simple terms, signaling molecules are endogenous compounds of chemical origin, which, through interaction with receptors, control the biochemical reactions taking place in target cells.
However, they have some features that you should be aware of. Signaling molecules are short-lived, highly biologically active, their actions are unique, and each of them can have several target cells at once.
By the way! Responses to one molecule of different target cells are often very different.
Nervous and humoral regulation
As part of the topic concerning the mechanisms of action of hormones on target cells, it will be useful to pay attention to this topic. It should be noted right away that the action of hormones is rather diffuse, and the nervous influence is differentiated. All because of their movement with the blood.
Humoral influence spreads rather slowly. The maximum speed that blood flow can reach varies from 0.2 to 0.5 m/s.
But despite this, the humoral influence is quite long-lasting. Itcan go on for hours, even days.
By the way, nerve endings often act as targets. But why is it always about a single neurohumoral regulation? Because the nervous system innervates the endocrine glands.
Target cell damage
One last thing to mention about this. The specifics of target cells and cell receptors have been studied above. It is worth completing the topic with information about which structural units are such a “magnet” for HIV, the most terrible virus.
For him, target cells are those on the surface of which there are CD4 receptors. This factor alone determines their interaction with the virus.
First, the varion binds to the cell surface, and reception occurs. Then they fuse with the membrane of the virus. It gets inside the cell. Subsequently, the nucleotide and PKN of the virus are released. The genome integrates into the cell. A certain time passes (latent period), and the translation of virus proteins begins.
All this is replaced by active replication. The process ends with the release of HIV proteins and varions from cells into the external environment of the body, which is fraught with unhindered infection of he althy cells. Unfortunately, this is a very sad example, but it clearly and intelligibly demonstrates the concept of "target" in this context.
