Target cells are structurally functional units that specifically interact with hormones using specific receptor proteins. The definition is generally clear, but the topic itself is very voluminous, and every aspect of it is certainly important. It is quite difficult to cover all the material immediately, so now we will focus only on the main points regarding target cells, their types and mechanism of action.
Definition
Target cells are a very interesting term. The prefix present in it is logically justified. Indeed, 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 that follows is directly related to metabolism.
How strongly the effect is realized determines the concentration of the hormone that has reacted with the target cell. This, however, is not the only key factor. The role is also played by the rate of hormone biosynthesis, the conditions for its maturation, and the specificity of the environment in which the cell contacts the carrier protein.
In addition, the antagonism or synergism of hormonal effects is reflected in the biochemical effect. For example, adrenaline 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 inhibits 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:
- Respond to physical factors (for example, light).
- Connect other molecules that carry signal regulators (neurotransmitters, hormones, etc.).
The latter function is 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 a cell or in the cytoplasm.
Receptors
It is necessary to tell about them separately. Target cell receptors are their specific chemical structures that contain complementary sites that bind to the 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 make a reservation that the receptor of any hormone is a protein with at least two domains (elements of the tertiary structure) that differ in structure and function.
What are their functions? Receptors work as follows: one of the domains binds a hormone, and the second produces a signal that is applicable to a particular intracellular process.
In steroid biologically active substances, everything happens a little differently. Yes, hormone receptors in this group also have at least two domains. Here only one of them carries out binding, and the second is associated with a specific DNA site.
Interestingly, in many cells there are so-called reserve receptors - those that do not participate in the formation of the biological response.
It's important to know
Studying the ways hormones act on target cells and other features of this topic, it is necessary to make a reservation that so far most of the receptors are not well understood. Why? Because their isolation and further purification are complex. But the content in the cells of each receptor is quite low.
However, the fact is known that hormones interact with receptors in a chemical-physical way. Between them, hydrophobic and electrostatic bonds are formed. When the receptor binds to the hormone, the receptor protein changes conformationally, as a result of which the complex is activated with the signal molecule.
Neurotransmitters
So-called 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, neurotransmitters also affect target cells.
More precisely, the "intermediaries" contact directly with the biochemical receptors, which were described above. The complex that these two substances form is capable of influencing the intensity of certain metabolic processes (through the goal of intermediaries or directly).
For example, one neurotransmitter causes an increase in the excitability of the target cell and the gradual depolarization of the postsynaptic membrane. Other "intermediaries" can have the exact opposite effect (inhibitory).
A number of substances block and activate receptors. These include prostaglandins, neuroactive peptides and amino acids. But in fact, there are much more substances that affect the process of information transfer.
Types of action of hormones on target cells
There are five of them. These types can be distinguished in the following list:
- Metabolic. It manifests itself in a change in the permeability of cell membranes, organoids, as well as the activity of intracellular enzymes and their synthesis. The hormones produced by the thyroid gland are distinguished by a pronounced metabolic effect.
- Corrective. This action affects the intensity of the functions provided by the target cells. Its severity depends on the reactivity and initial state. As an example, we can recall the effect of adrenaline on heart rate.
- Kinetic. When such an effect is exerted, the target cells pass from a calm state to an active one. A striking example is the reaction of uterine muscles to oxytocin.
- Morphogenetic. It consists in changing the size and shape of target cells. Growth hormone, 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, we must make a reservation that if they are water-soluble, then they influence without penetrating inside - that is, from the surface of the cytoplasmic membrane.
Here are the steps involved in this process:
- The formation on the membrane surface of GRK (hormone-receptor complex).
- Subsequent activation of enzymes.
- Formation of secondary intermediaries.
- The formation of protein kinases of a certain group (enzymes that modify other proteins).
- Activation of protein phosphorylation.
The described process, by the way, is correctly called reception.
Interactions with Fat Soluble Hormones
Or, as they are called most often - with steroidal. In this case, another effect of hormones on target cells appears. Because steroids, unlike water-soluble substances, still penetrate inside.
Phasedly, it looks like this:
- The steroid hormone is in contact with the membrane receptor, after which GRK is transferred to the cell.
- The substance then binds to the cytoplasmic protein receptor.
- After this, GRK is transferred to the core.
- Interaction with the third receptor is carried out, which is accompanied by the formation of GRK.
- Following GRK, it binds to DNA and, of course, to a chromatin acceptor.
Having studied this pathway of the action of hormones on target cells, it can be understood that GRK has been present in the nucleus for quite some time. Therefore, all physiological effects occur several hours after the start of the process.
Signal Recognition
And a couple of words are worth saying too. The signals entering the body are of two types:
- External What does it mean? That signals to the cell come from the external environment.
- Internal Signals are formed and then act in the same cell. Often, signals are metabolites that play the role of allosteric inhibitors or activators.
Regardless of type, they have the same tasks. They can be distinguished in the following list:
- Exclusion of the so-called idle metabolic cycles.
- Maintaining proper homeostasis.
- Intercellular and internal coordination of metabolic processes.
- Regulation of the processes of formation and further use of energy.
- Adaptation of the body to environmental changes.
In simple terms, the signaling molecules are endogenous compounds of chemical origin, which, thanks to the interaction with the receptors, provide control of the biochemical reactions that take place in the target cells.
However, they do have features worth knowing about. Signal molecules do not last long, are characterized by high biological activity, their actions are unique, and each of them can have several target cells at once.
By the way! Reactions to a single molecule of different target cells are often very different.
Nervous and humoral regulation
As part of the topic regarding the mechanisms of action of hormones on target cells, it will be useful to pay attention to this topic. It is immediately worth noting that the action of hormones is rather diffuse, and the nervous effect is differentiated. All because of their movement along with blood.
The humoral effect spreads rather slowly. The maximum speed that blood flow can reach varies from 0.2 to 0.5 m / s.
But, in spite of this, the humoral influence is quite lasting. It can last for hours, even days.
By the way, often in the role of targets are nerve endings. But why is it always about a single neurohumoral regulation? Because the nervous system innervates the endocrine glands.
Target cell damage
Finally, mention should be made of this. The specificity of target cells and cell receptors has been studied above. To complete the topic is information about which structural units are such a “magnet” for HIV - the worst virus.
For him, the 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 surface of the cell, reception occurs. Then they merge with the membrane of the virus. It penetrates the cell. Following this, the nucleotide and ILV of the virus are released. The genome integrates into the cell. A certain time passes (latent period), and the translation of the 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 healthy cells. Unfortunately, this is a very sad example, but it clearly and intelligibly demonstrates the concept of “target” in the context under consideration.