Tasks of hormones | Hormones

Tasks of hormones

Hormones are messenger substances of the body. They are produced by various organs (for example thyroid, adrenal gland, testicles or ovaries) and released into the blood. In this way they are distributed to all areas of the body.

The different cells of our organism have different receptors to which special hormones can bind and thus transmit signals. In this way, for example, the circulation or metabolism is regulated. Some hormones also have an effect on our brain and influence our behaviour and our sensation.Some hormones are even only found in the nervous system and mediate the transfer of information from one cell to the next at the so-called synapses.

a) Cell surface receptors: After the hormones belonging to the glycoproteins, peptides or catecholamines have bound to their specific cell surface receptor, a multitude of different reactions take place in the cell one after the other. This process is known as a signaling cascade. Substances involved in this cascade are called “second messengers”, in analogy to the hormones called “first messengers”.

The atomic number (first/second) refers to the sequence of the signal chain. At the beginning, the first messengers are the hormones, the second messengers follow in a time-delayed manner. The second messengers include smaller molecules such as cAMP (cyclic adenosine monophsophate), cGMP (cyclic guanosine monophosphate), IP3 (inositol triphosphate), DAG (diacylglycerol) and calcium (Ca).

The cAMP-mediated signaling pathway of a hormone requires the involvement of so-called G-proteins coupled to the receptor. G-proteins consist of three subunits (alpha, beta, gamma), which have bound a GDP (guanosine diphosphate). When hormone receptor binding occurs, GDP is exchanged to GTP (guanosine triphosphate) and the G-protein complex decays.

Depending on whether they are stimulatory (activating) or inhibitory (inhibiting) G-proteins, a subunit now activates or inhibits an enzyme called adenylyl cyclase. When activated, the cyclase produces cAMP; when inhibited, this reaction does not occur. cAMP itself continues the signaling cascade initiated by a hormone by stimulating another enzyme, protein kinase A (PKA).

This kinase is able to attach phosphate residues to substrates (phosphorylation) and thus initiate the activation or inhibition of downstream enzymes. Overall, the signaling cascade is amplified many times over: a hormone molecule activates a cyclase, which – when acting as a stimulator – produces several cAMP molecules, each of which activates several protein kinases A. This reaction chain is terminated by the aggregation of the G-protein complex after decomposition of the GTP to GDP and by enzymatic inactivation of the cAMP by the phosphodiesterase.

Substances altered by phosphate residues are freed from the attached phosphate with the help of phospatases and thus reach their original state. The second messenger IP3 and DAG are generated simultaneously. Hormones activating this pathway bind to a Gq-protein coupled receptor.

This G-protein, also consisting of three subunits, activates the enzyme phospholipase C-beta (PLC-beta) after hormone receptor binding, which cleaves off the cell membrane IP3 and DAG. IP3 acts on the cell’s calcium stores by releasing the calcium it contains, which in turn initiates further reaction steps. DAG has an activating effect on the enzyme protein kinase C (PKC), which provides various substrates with phosphate residues.

This reaction chain is also characterized by an amplification of the cascade. The end of this signaling cascade is reached with self-deactivation of the G-protein, the degradation of IP3 and the help of phosphatases. b) Intracellular receptors: Steroid hormones, calcitriol and thyroid hormones have receptors located in the cell (intracellular receptors).

The receptor of steroid hormones is present in an inactivated form, since so-called heat shock protein (HSP) is bound. After hormone binding, these HSP are split off so that the hormone-receptor complex can migrate into the cell nucleus. There the reading of certain genes is made possible or prevented, so that the formation of proteins (gene products) is either activated or inhibited.

Calcitriol and thyroid hormones bind to hormone receptors, which are already located in the cell nucleus and are transcription factors. This means that they initiate gene reading and thus protein formation. Hormones are integrated into so-called hormonal control loops, which control their formation and release.

An important principle in this context is the negative feedback of the hormones. Feedback means that the response (signal) triggered by the hormone is fed back to the hormone releasing cell (signal generator). Negative feedback means that when a signal is given, the signal generator releases less hormones and thus the hormonal chain is weakened.Furthermore, the size of the hormonal gland is also influenced by the hormonal control circuits and thus adapted to the requirements.

This is done by regulating cell number and cell growth. If the number of cells increases, this is called hyperplasia, it decreases as hypoplasia. Increased cell growth results in hypertrophy, whereas cell shrinkage results in hypotrophy.

The hypothalamus-pituitary system is an important hormonal control circuit. The hypothalamus represents a part of the brain, the pituitary gland is the pituitary gland, which is divided into an anterior lobe (adenohypophysis) and a posterior lobe (neurohypophysis). Nervous stimuli of the central nervous system reach the hypothalamus as “switchboard”.

The hypothalamus in turn unfolds its effect on the pituitary gland through liberine (releasing hormones) and statine (release-inhibiting hormones). Liberine stimulates the release of pituitary hormones, statins inhibit them. Subsequently, hormones are released directly from the posterior lobe of the pituitary gland.

The anterior lobe of the pituitary gland releases its messenger substances into the blood, which then travel via the blood circulation to the peripheral end organ, where the corresponding hormone is secreted. For each hormone there is a specific liberin, statin and pituitary hormone. The hormones of the posterior lobe of the pituitary gland are the liberin and statin of the hypothalamus and the downstream hormones of the anterior lobe of the pituitary gland are the liberin and statin: The path of the hormones begins in the hypothalamus, whose liberins act on the pituitary gland.

“intermediate hormones” produced there reach the peripheral hormone formation site, which produces the “end hormones”. Such peripheral sites of hormone formation are, for example, the thyroid gland, the ovaries or the adrenal cortex. The “end hormones” include the thyroid hormones T3 and T4, estrogens or the mineral corticoids of the adrenal cortex.

In contrast to the path described above, there are also hormones independent of this hypothalamic-pituitary axis, which are subject to different regulatory circuits. These include:

  • ADH = antidiuretic hormone
  • Oxytocin
  • Gonadotropin releasing hormone (Gn-RH) ? Follicle Stimulating Hormone (FSH)luteinizing Hormone (LH)
  • Thyreotropin Releasing Hormones (TRH) ?

    ProlactinThyroid Stimulating Hormone (TSH)

  • Somatostatin ? inhibits prolactinTSHGHACTH
  • Growth Hormone Releasing Hormones (GH-RH) ? Growth Hormone (GH=Growth Hormone)
  • Corticotropin Releasing Hormones (CRH) ? Adrenocorticotropic hormone (ACTH)
  • Dopamine ? inhibits Gn-RHprolactin
  • Hormones of the pancreas: insulin, glucagon, somatostatin
  • Kidney hormones: Calcitriol, Erythropoietin
  • Hormones of the parathyroid gland: Parathyroid hormone
  • Further hormones of the thyroid: Calcitonin
  • Hormones of the liver: Angiotensin
  • Hormones of the adrenal medulla: adrenaline, noradrenaline (catecholamines)
  • Hormone of the adrenal cortex: Aldosterone
  • Gastrointestinal hormones
  • Atriopeptin = atrial natriuretic hormone of the muscle cells of the atria
  • Melatonin of the pineal gland (epiphysis)