Hormones: definition, structure & examples

Without the hormonal form of information transmission, a meaningful interaction of the cells in an organism would not be possible. As signal transmitters, hormones coordinate metabolism, growth, reproduction and many other processes in the body. Important messenger substances are present in the blood in very low concentrations. But even this low concentration of hormones is sufficient for their effects to develop.

Hormones – forms of hormone secretion

Hormones can be released and reach their target cell in a number of ways. There are the following options:

1. endocrine secretion
2. paracrine secretion
3. autocrine secretion

1. Endocrine Secretion

During endocrine secretion, hormones are produced by endocrine glands and reach their target cells via the bloodstream. The secreted hormones are called glandular hormones (Latin: glandula = gland). In most cases, however, the secretion does not take place continuously, but intermittently (pulsatile) and follows a certain biological rhythm (circadian rhythm).

2. Parakine secretion

During paracrine secretion, the hormones are released to the cells of the neighboring tissue. Therefore, hormones secreted in this way are also referred to as tissue hormones. In contrast to the glandular hormones, they are not usually formed by endocrine glands, but in special individual cells. The tissue hormones include, for example, prostaglandins. They are formed in various body cells and lead to pain when there is inflammation. They can be inhibited in their synthesis by acetylsalicylic acid, an active ingredient in painkillers.

3. Autocrine secretion

In the case of autocrine secretion, the signal molecule formed acts on the hormone-producing cell itself. This is the case with insulin, for example, because the insulin-producing B cells in the pancreas have insulin receptors and can therefore also be influenced by insulin themselves. As a rule, however, it is not glandular hormones that are secreted autocrine. Instead, there are for example growth factorsthat act on the cell that produced them. This property of growth factors is important for tissue differentiation during embryonic development.

Figure 1: Forms of hormone secretion

Classification of hormones

Hormones can be classified based on their properties:

• Classification of hormones based on their secretion modes:Autocrine, paracrine and endocrine secretion (as discussed in the section above).
• Classification of hormones into glandular and Aglandular Hormones: Glandular hormones, such as insulin, are made by endocrine glands. Aglandular hormones, on the other hand, are formed in special, endocrine-active individual cells. This includes, among other things, gastrin, which is produced by the G-cells of the stomach.
• Classification of hormones based on their chemical properties.

Hormones – Properties

Not all hormones have a similar structure and chemical properties. Instead, they can vary greatly in their structure.

Therefore, hormones are classified based on their reaction with water

• hydrophilic (water-soluble) hormones and
• lipophilic (fat-soluble) hormones

assigned.

Hydrophilic hormones – peptide hormones

Peptide hormones consist of individual amino acids and are water-soluble. Therefore, they can be stored in the vesicles of cells (bubbles surrounded by a membrane). These types of hormones are broken down in the blood plasma and in the kidneys, where the hormones are broken down by certain enzymes.

The hydrophilic hormones include, among others

• insulin
• vasopressin
• glucagon
• somatotropin

Lipophilic hormones – steroid hormones and thyroid hormones

Steroid hormones are derived from fat-soluble cholesterol and are therefore fat-soluble themselves. Since the membrane of storage vesicles would simply let such lipophilic substances through, steroid hormones can hardly be stored. The breakdown of steroid hormones takes place in the liver.

The steroid hormones include:

• testosterone
• cortisol
• estrogens.

The hormones of the thyroid gland (triiodothyronine and thyroxine) are also lipophilic, but do not belong to the steroid hormones since they are made up of amino acids and are not derived from cholesterol.

The chemical properties of the hormones are also important in terms of their mechanism of action. Because only fat-soluble hormones can penetrate the lipophilic cell membrane, while peptide hormones cannot pass through the cell membrane due to their hydrophilic nature.

hormones effect

As already mentioned, endocrine hormones reach their target organ via the bloodstream. In order to develop their effect there, the hormones bind to receptors. Depending on whether the hormone is water-soluble or lipophilic, the hormone receptor is located on the cell surface (for hydrophilic hormones) or inside the cell (for lipophilic hormones). Within the cell, the receptor can be located either in the cytoplasm or in the nucleus.

In the case of hydrophilic hormones, binding of the hormone to the receptor on the cell membrane results in a change in the conformation of the receptor. In order for the information to reach the cell, the change in conformation inside the cell triggers a kind of chain reaction in which second messengers play an important role.

An example of this type of signal transmission can be found in the peptide hormone glucagon. It uses cyclic adenosine monophosphate, cAMP for short, as a second messenger. To do this, glucagon binds to a G protein-coupled receptor, which changes its conformation as a result of the binding. As the name G protein-coupled receptor already suggests, a so-called G protein is coupled to the receptor. This G protein is activated by the conformational change and stimulates the enzyme adenylate cyclase.

This enzyme then converts ATP into cAMP. In response to the increase in cAMP levels inside the cell, protein kinase A is activated. In the case of glucagon, protein kinase A then activates the enzyme glycogen phosphorylase, which stimulates glycogen breakdown. This is how glucagon acts as a hormone to break down sugar.

Figure 2: Signal transduction using cAMP as second messenger

In contrast to hydrophilic hormones, lipophilic hormones can pass through the cell membranes unhindered and therefore do not require a second messenger. In the case of the lipophilic hormones, the hormones form complexes together with the receptors, which affect transcription and translation in the cell nucleus and thus influence gene expression.

This is the case, for example, with testosterone, which binds to the androgen receptor in the cell nucleus. The complex formed during binding acts as a transcription factor and thus influences the synthesis of certain proteins.

Hormones – examples and functions

Hormones are not only chemically diverse, but also in terms of their tasks:

• the adaptation of the organism to changing environmental conditions
• the regulation of growth
• the reproduction
• the electrolyte and water balance
• the metabolism.

growth hormone

Growth hormone is also known as somatotropin. It is a peptide hormone that stimulates the growth of various tissues how bones and muscles are affected. In addition, it represents energy ready by giving it the blood sugar increased and the fat loss stimulates. At the same time, it promotes build-up of proteins.

thyroid hormones

The thyroid produces three main hormones:

• Thyroxine (T4)
• Triiodothyronine (T3)
• calcitonin.

The first two hormones are considered thyroid hormones in the classic sense. In contrast to calcitriol, they are formed by the thyrocytes (epithelial cells of the thyroid gland), while calcitonin is produced by the C cells and also differs significantly from T3 and T4 in terms of function and structure.

The job of the thyroid hormones is to increase basal metabolic rate. You increase the breathing and heart ratemobilize fat reserves and increase the absorption of carbohydrates as part of the digestion.

hormones of the adrenal glands

The adrenal gland also produces several important hormones. A distinction is made between those that are formed in the adrenal medulla and those that are synthesized in the adrenal cortex:

• In the adrenal medulla are formed:

• In the adrenal cortex are formed:
  • glucocorticoids (cortisol)
  • aldosterone
  • androgens.

The catecholamines adrenaline, noradrenaline, and dopamine are special hormones because, as neurotransmitters, they create a link between the endocrine and nervous systems. They are involved in the transmission of stimuli from nerve cell to nerve cell both in the central nervous system and in the autonomic nervous system. as stress hormones they are released as part of the stress reaction and cause, among other things, an increase in heart rate.

The hormones of the adrenal cortex are steroid hormones. Cortisol is also a stress hormone. It works immunosuppressive (it suppresses the immune system), which is why it is used in a synthetically produced form in medicine, for example after organ transplants. Aldosterone is important for the reabsorption of sodium and water in the kidneys and is thus also involved in the regulation of blood pressure. The androgens are the male sex hormones.

pancreatic hormones

Pancreatic hormones include:

• glucagon
• insulin
• somatostatin.

Insulin and glucagon are the key hormones involved in the regulation of blood sugar levels. The two hormones behave as antagonists to each other: insulin lowers the blood sugar level, while glucagon increases it. Somatostatin works by inhibiting the release of the growth hormone somatotropin.

gonadal hormones

The most important gonadal hormones, i.e. hormones of the gonads (testicles and ovaries), include the following sex hormones:

• estrogen
• progesterone
• Testosterone.

You all belong to them steroid hormones. The effects of estrogen are manifold. Estrogens are important in controlling female cyclethey influence that breast growth and much more. Progesterone plays an important role in the implantation of fertilized eggs in the lining of the uterus and in maintaining the uterus pregnancy.

testosterone is relevant, among other things, for the training of male sex organsthe education…