Last updated on November 19, 2018 at 17:16
- 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 be transported through the blood, as part of endocrine action, they can be transported to neighbouring cells through paracrine action, or affect the same cell that produced through autocrine action.
- Many different types of hormones exist: peptide hormones, catecholamines, eicosanoids, steroids, vitamin D derivatives, retinoid hormones, nitric oxide and thyroid hormones
Peptide hormones are simple peptide chains. Examples of these are insulin and glucagon. They are produced by proteolytic cleavage, and bind to receptors in the plasma membrane. 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. All hormones synthesized from the pituitary and hypothalamus are peptide hormones.
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 are hormones that are derived from polyunsaturated fatty acids, by the action of phospholipase A2. The biggest groups of eicosanoids are prostaglandins, leukotrienes and thromboxanes, all produced from the polyunsaturated fatty acid arachidonic acid. They act on neighbouring cells (paracrine). They act through plasma membrane receptors.
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.
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.
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.
30. Mitochondrial protein synthesis, mitochondrial genome
32. Epinephrine and glucagon