31. Hormones

Last updated on January 11, 2020 at 13:05

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
    • These hormones work by intracellular signalling
    • Steroid hormones
    • Vitamin D-derivative hormones
    • Vitamin A-derivative hormones
    • Thyroid hormones

Peptide hormones

Peptide hormones like insulin and glucagon 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

The three catecholamines are dopamine, norepinephrine and epinephrine. They are synthesized in the brain and in the adrenal glands. They are all synthesized from tyrosine.

Eicosanoids

Eicosanoids are hormones that are derived from phospholipids in the membrane. These phospholipids are converted into arachidonic acid by phospholipse 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.

Steroids

Steroids regulate gene expression, and are synthesized in mitochondria from cholesterol (which is synthesized in cytosol and ER). Steroids are endocrine hormones, and because they are lipid-soluble, can travel freely through the cell membrane without a transporter. They instead act on receptors directly on the nucleus, so called type I nuclear receptors.

The four types of steroid hormones are glucocorticoids, like cortisol, mineralocorticoids, like aldosterone, androgens, like testosterone, and estrogens, like estradiol. Cortisol regulates carbohydrate metabolism, aldosterone regulates electrolyte concentration, while the sex hormones affect sexual development and sexual behaviour.

Hormone positive breast cancer is treated with tamoxifen, a steroid drug. Asthma drugs are also steroid drugs that relax smooth muscle in the respiratory tract.

Vitamin D derivatives

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 act through type II nuclear receptors. The skin, kidney and liver are all involved in the synthesis of calcitriol.

Retinoic acid hormone

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

These hormones are synthesized in the thyroid gland from thyroglobulin and requires iodine. They act through type II nuclear receptors, and stimulate energy metabolism in the liver and the muscle.

Nuclear receptors

There are two types of nuclear receptors, receptors that affect the DNA somehow.

Type I nuclear receptors are actually located in the cytoplasm. When the hormone enters the cytoplasm, it will bind to the nuclear receptor, which will create a dimer with another nuclear receptor with a hormone. This dimer of two nuclear receptors and two hormones then passes the nuclear envelope through a nuclear pore, and then binds to DNA to begin DNA transcription.

Type II nuclear receptors are found in the nucleus. They exist in a complex with a retinoid X receptor (RXR), and are bound to a corepressor, a molecule that represses its activity. After the hormone enters the nucleus through a nuclear pore, it will bind the nuclear receptor/RXR complex, which will cause the corepressor to dissociate. The complex will then bind RNA polymerase, which will begin transcription.

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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