31. Hormones

Last updated on March 11, 2020 at 19:34

Summary

  • Different systems in the body use different chemical messengers to transmit messages between cells. The nervous system uses neurotransmitters, the endocrine system uses hormones, and the immune system uses cytokines
  • Hormones can act on cells in three different “ways”
    • They can be transported through the blood, acting on cells that are far away. This is called endocrine action
    • They can act on neighbouring cells. This is called paracrine action
    • They can affect the same cell that produced them. This is called autocrine action.
  • Many different types of hormones exist
    • Peptide hormones
      • Insulin
      • Glucagon
      • Vasopressin
      • Growth hormone
      • etc.
    • Catecholamines
      • Epinephrine
      • Norepinephrine
      • Dopamine
    • Eicosanoids
      • Prostaglandins
      • Thromboxanes
      • Leukotrienes
    • Steroid hormones
      • Sex hormones
        • Testosterone
        • Oestrogen
      • Cortisol
      • Aldosterone
    • Vitamin D derivatives
    • Retinoid hormones (vitamin A derivatives)
    • Gas hormones
      • Nitric oxide
    • Thyroid hormones
      • T3
      • T4
  • Most hormones bind to a receptor on the surface of the cell, but some diffuse across the cell membrane and bind to receptors inside the cytoplasm, like:
    • Steroid hormones
    • Vitamin D-derivative hormones
    • Vitamin A-derivative hormones
    • Thyroid hormones

Hormones

Hormones are signal molecules produced by the body. They are produced by specialized so-called endocrine cells. Hormones are produced by the endocrine cells in response to a certain stimulus. The job of the hormone is to “transmit” the signal of this stimulus to other cells in the body, which allows many cells to act in a coordinated fashion.

We can classify hormones according to which cells they act on, generally speaking:

  • Endocrine hormones are released into the bloodstream by the endocrine cells
    • Endocrine hormones travel with the blood and can therefore act on cells throughout the whole body
    • Most important hormones are endocrine
  • Paracrine hormones are produced by endocrine cells, and they will reach neighbouring cells by diffusion
    • Paracrine hormones are not released into the bloodstream
    • They only act on cells very close to the endocrine cells
  • Autocrine hormones are produced by endocrine cells and act on the same endocrine cell
    • These are not super-important for us

We can also classify hormones according to their structure:

  • Peptide hormones are peptide chains made up of many amino acids
    • Insulin
    • Glucagon
    • Vasopressin
    • Oxytocin
  • Amine hormones are derived from a single amino acid
    • Catecholamines
    • Thyroid hormones
  • Steroid hormones have a special steroid structure
    • They are all synthesized from cholesterol
    • Testosterone
    • Progesterone
    • Oestrogen
    • Glucocorticoids
    • Mineralocorticoids

Hormones act on target cells by binding to a certain protein on or in the target cell. These proteins are called receptors. Each hormone has a unique receptor (with a few exceptions). After the hormone has bound to its receptor on the target cell, it will trigger a cascade of intracellular processes which ultimately change the behaviour of the target cell, which is the whole point of hormones.

Most hormone receptors are located on the surface of the cells, but some hormone receptors are located inside the cells or even inside the nuclei. The hormones whose receptor are inside the cells can freely diffuse across the cell membrane, so it is no problem for them that the receptor is on the inside of the cell. We call these receptors cell surface receptors or plasma membrane receptors and intracellular receptors.

More about hormone receptors in topic 34.

Peptide hormones

  • Insulin
  • Glucagon
  • Anterior pituitary hormones
    • Growth hormone (GH)
    • Thyroid-stimulating hormone (TSH)
    • Adrenocorticotropic hormone (ACTH)
    • Follicle-stimulating hormone (FSH)
    • Luteinizing hormone (LH)
    • Prolactin
  • Posterior pituitary hormones
    • Oxytocin
    • Vasopressin/Anti-diuretic hormone (ADH)
  • Hypothalamic hormones
    • Corticotrophin-releasing hormone (CRH)
    • Dopamine
    • Growth hormone-releasing hormone (GHRH)
    • Somatostatin
    • Gonadotrophin-releasing hormone (GnRH)
    • Thyrotrophin-releasing hormone (TRH)

Peptide hormones are simple peptide chains. They bind to receptors on the surface of the cell. Peptide hormones can be very similar to one another, but still have very different effects. Oxytocin and vasopressin, for example, differ only by two amino acids, but have very different effects. All hormones synthesized from the pituitary and hypothalamus are peptide hormones.

Catecholamines

  • Dopamine
  • Epinephrine/adrenaline
  • Norepinephrine/noradrenaline

They are synthesized in the brain and in the medulla of the adrenal glands. They are amine hormones because they’re synthesized from tyrosine.

Norepinephrine and epinephrine bind to so-called α-adrenergic and β-adrenergic receptors. These hormones are released in periods of stress.

Eicosanoids

  • Prostanoids
    • Prostaglandins
    • Thromboxanes
  • Leukotrienes

Eicosanoids are hormones that are derived from phospholipids in the membrane. These phospholipids are converted into arachidonic acid by the enzyme phospholipase A2. Arachidonic acid can then be converted by cyclooxygenase into prostaglandins or thromboxanes, or by lipoxygenase into leukotrienes.

They act on neighbouring cells, or in other words they’re paracrine hormones. They bind to cell surface receptors.

Steroid hormones

  • Sex hormones
    • Progesterone
    • Testosterone
    • Oestradiol
  • Mineralocorticoids
    • Aldosterone
  • Glucocorticoids
    • Cortisol

Steroid hormones don’t act on cell surface receptors. Instead, they freely diffuse across the cell membrane and bind to intracellular receptors. Different steroid hormones are synthesized in different cells of the body, but all are synthesized in mitochondria from cholesterol (which is synthesized in cytosol and ER).

The intracellular receptor of steroid hormones are so-called type I nuclear receptors.

Cortisol is a stress hormone which is produced by the cortex of the adrenal medulla. It stimulates gluconeogenesis, suppresses the immune system, increases insulin resistance, and increases the blood pressure. Drugs which have cortisol-like effects are widely used in medicine for their immune system-suppressing effects.

Aldosterone is a hormone which regulates the renal balance of sodium, water and potassium.

The sex hormones affect sexual development, sexual behaviour, muscle growth, and other stuff.

Vitamin D derivatives

  • Calcitriol

Calcitriol is the main vitamin D derivative hormone. It is important in calcium homeostasis, the balance between Ca2+ in deposition and Ca2+ mobilization from bone. They don’t bind to cell surface receptors but act through type II nuclear receptors. The skin, kidney and liver are all involved in the synthesis of calcitriol.

Retinoic acid hormone

  • Retinoic acid

Retinoic acid is synthesized in the liver from vitamin A. It regulates gene expression, and acts through type II nuclear receptors. It is important in regulating growth, survival and differentiation of cells.

Thyroid hormones

  • Triiodothyronine (T3)
  • Thyroxine (T4)

Thyroid hormones are synthesized in the thyroid gland from thyroglobulin (which is a derivative of tyrosine) and requires iodine. They act through type II nuclear receptors, and stimulate energy metabolism in the liver and the muscle.

A table of the hormones, just so you can see that I’m not lying.

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30. Mitochondrial protein synthesis, mitochondrial genome

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32. Epinephrine and glucagon

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