My article from last week focused on the key components of the endocrine system. Following the same idea, I thought it would be ideal to do a follow up article on the classification of hormones and even mention some of their functions along the way.
Hormones are small chemical messengers that travel through our bloodstream to target cells. Target cells are the specific cells that hormones act on. A single hormone can have many target cells, but each one can respond differently to the hormone.
Hormones work by binding to receptors on or within the cell and changing the activity of that particular cell. It is important to note that hormones do not initiate any reactions in their target cells. There are two main ways to classify hormones: structure and solubility. However, they can also be separated based on whether they are tropic or non-tropic.
Under this method of classification, there are four groupings: steroids, peptides, amino acid and fatty acid.
Steroid hormones are derived from cholesterol. The sex hormones (androgens, estrogens and progesterone) and hormones produced in the adrenal glands. Adrenal gland hormones include the mineralcoritcosteriods and gluccorticosteriods. Steroid hormones have a few important functions in the body including water balance, sexual development and stress response.
The next type is the amino acid derivative hormones. They are commonly derived from tyrosine and tryptophan. There are two types of tyrosine derived hormones: thyroid hormone and catecholamines.
Thyroid hormone is one of the most important in the body because it regulates the development of organs and metabolism. Norepinephrine and epinephrine are catecholamines. Both are stress hormones and are also neurotransmitters. They are part of the flight or fight response. Both increase heart rate, dilate blood vessels and cause the release of glucose during times of stress.
Tryptophan is a precursor to serotonin and melatonin. Serotonin is mostly found in our GI tract and regulates movement in our intestines. However, it can also be found in the platelets of our blood. Serotonin is associated with mood and low levels often result in depression. Serotonin is also part of appetite and can make you sleepy.
Hormones can also be derived from peptides. Prohormones are the precursors for peptide hormones. These prohormones are synthesized in the endoplasmic reticulum or ER for short. Like all good proteins, they need to be folded into the proper configuration in order to be functional.
Peptide hormones are stored in the vesicles of the cell until the proper stimuli signals release into the bloodstream. These hormones can be very small (a few amino acids in length) or large (around 100 amino acids). An example of a smaller peptide hormone is TSH (thyroid stimulating hormone) and a larger peptide hormone is insulin.
Other peptide hormones of honorable mention are prolactin (aids in lactation), growth hormone and vasopressin (water regulation), which are produced by the pituitary gland. Another, less commonly known example, is atrial natiuretic peptide (ANP), which is produced by the heart. ANP regulates homeostasis of the circulatory system. It is released in response to high blood pressure and dilation of the atrium.
There is still one type of structural classification left to discuss: hormones derived from fatty acids.
Hormones that are arrived from fatty acids are called eicosanoids. They are synthesized from a 20-carbon amino acid called arachidonic acid. They degrade very easily, which simply means that they do not hang around long. They also do not stray far from their production site.
A unique characteristic in that they are produced and secreted by nearly every cell in the body instead of just one gland. Eicosanoids have various important roles in the body including inflammation, blood pressure and blood clotting.
Prostaglandins are classified as eicosanoids and have a wide variety of functions ranging from uterine contractions to bronchodilation. They even have a hand in inflammation and fever. In fact, aspirin acts on prostaglandins to reduce pain and fever.
Lipid Soluble vs Water Soluble Hormones
Lipid soluble hormones are able to pass right through the target cell’s membrane. They work by binding to receptors inside the cell. This binding activates the hormone, which then binds to a portion of DNA inside the cell’s nucleus. Binding to the DNA causes genes to turn enzyme activity on or off, which alters the activity of the target cell.
Steroid derived hormones are lipid soluble. Lipid soluble hormones act directly on the cell.
Water soluble hormones, however, act indirectly on target cells. Since the cell membrane has a lipid bi-layer, it is hydrophobic, or water fearing. This simply means that anything that is water soluble is not getting through. Unlike lipid soluble hormones, water soluble hormones have to bind to receptors on the surface of the target cell. Once the hormone is bound to the receptor, enzyme activity inside the cell is altered. Depending on the hormone, enzyme activity is increased or decreased. Water soluble hormones include those that are derived from amino acids and polypeptide hormones.
Trophic Hormones and Tropic vs Non-tropic Hormones
Trophic hormones stimulate the growth of tissues and organs. Trophic hormones include hormones of the anterior pituitary: TSH and ACTH. TSH stimulates the thyroid and ACTH stimulates the adrenal glands. TSH and ACTH can also be considered tropic hormones. Tropic hormones act on endocrine glands rather than target cells.
Other examples include LH (lutenizing hormone) and FSH (follicle stimulating hormone). LH acts on the gonads to produce eggs and sperm while FSH stimulates the gonads to produce steroids. Non-tropic hormones behave in an opposite manner. They do not act on endocrine glands, but on target cells.
And there you have it: the classification of hormones and some of the most important hormones in the body. It isn’t too complicated to keep track of them once you get past the structural classification and sub classification in some cases. After that, it is all downhill.