HOW HORMONES ARE MADE
Hormones are produced by specialized cells within glands or organs from the endocrine system: the hypothalamus, pineal and pituitary glands (all located in the brain), as well as the thyroid, parathyroids, adrenal glands, reproductive glands (ovaries and testes) and the pancreas. However, other organs that are not traditionally considered part of the endocrine system also contain specialized, hormone-producing cells. Specialized cells in the intestine, for example, produce secretin; this helps to regulate water absorption in the gut which (ideally) keeps waste moving along smoothly. All these cells must build and store one or more different hormones and secrete them when they receive the right signal.
Cells are stimulated to secrete hormones through one of three pathways: hormonal, humoral and neural. As the name suggests, hormonal stimuli are chemical in nature – for example, cells in the thyroid are stimulated to produce the hormones triiodothyronine and thyroxine when the anterior pituitary gland releases thyroid-stimulating hormone. Humoral stimulation is from a ‘sensor’ associated with the endocrine gland itself. For example, cells in the parathyroid gland have calcium-sensing receptors on their surface to enable them to detect calcium levels in the blood.
If levels are low, these cells will release parathyroid hormone (PTH), which affects a number of target cells in the body in order to increase calcium levels. Neural stimuli generate a rapid release of hormone. For example, secreting cells in the adrenal medulla are stimulated by nerve impulses to release both adrenaline (epinephrine) and noradrenalaine (norepinephrine); these prepare the body for the ‘fight or flight response by, for example, increasing heart rate.
Hormone-secreting cells do not wait until they receive a stimulus to start building hormones. Instead it is an ongoing process, in which cells constantly construct different useful molecules – a process known as biosynthesis. This broadly involves either modifying simple molecules or combining them with other simple molecules in order to create larger, more complex molecules. The blueprint for constructing all the simple molecules exists within the cells’ DNA.
Once a hormone has been built, it may temporarily be stored in a special compartment within the cell until it is needed. When the secreting cell receives the signal to release its hormone, the compartment moves to the cell’s plasma membrane. Here the hormone molecules are released through a process known as exocytosis.
Hormones cause large-scale changes by affecting many cells, often in different parts of the body, at the same time. How effective a hormone may be depends on its concentration levels in the blood as well as how receptive its target cells are to its presence.
After a hormone has been secreted from a cell, it travels through the fluids surrounding the cell to the nearest capillary. The hormone molecules are small enough to pass through the capillary wall and so enter the bloodstream. The amount of hormone secreted into the bloodstream depends on the strength and duration of the stimulus received by the secreting cell.
Nearly 50 per cent of your blood is actually water. However, fat-loving hormones (steroids and thyroid hormones) are not particularly happy in this environment; they need some help from transport proteins that circulate in the blood. These proteins bind to the hormones, essentially ‘covering up’ the hydrophobic ends of the hormones with their own water-loving structure. This enables the hormones to move more easily through this aqueous environment.
Some transport proteins are highly specific. Sex hormone-binding globulin (SHBG), for example, binds only with the sex hormones testosterone and ostrogen. Other transport proteins, such as albumin, will bind to any hydrophobic hormone. Generally the more specific the bond, the tighter it is, and so the further the transport protein will carry the hormone.
As soon as hormones enter the bloodstream they are not only diluted, but are at risk of being broken down through the body’s natural metabolic pathways (see half-life, page 29). As a result humans (along with other animals) have evolved a couple of direct transport routes. These ensure some of the more important hormones reach their destinations without getting lost in the circulatory (blood) system. The first of these direct routes is between the hypothalamus and the anterior pituitary (part of the pituitary gland that regulates processes such as growth and reproduction). Known as the hypothalamic-hypophyseal portal circulation, this route ensures that the hormones secreted by the hypothalamus (such as growth hormone) reach the anterior pituitary undiluted.
The second direct route is called the hepatic portal circulation. It carries blood directly from the gastrointestinal tract and spleen to the liver via the portal vein, hence its name. This fast lane goes direct to the liver, meaning that the two hormones responsible for regulating blood sugar levels, insulin and glucagon, are sent express to the major organ involved in storing and releasing glucose. After reaching the liver the hormones continue to circulate through the rest of the body.
Hormones are divided into two types: those that act as primary messengers, where the hormone itself travels directly into a cell to create a response, and those that bind to a receptor on the target cell’s surface, causing the cell to release proteins (known as secondary messengers) to create a response.
PRIMARY MESSENGERS
Unlike water-loving hormones, fat-loving hormones (such as steroids and thyroid hormones) are more than happy to cross the lipid membrane in order to enter the cell. As a result, receptors for these hormones are found within the target cells – either in the cytoplasm or in the nucleus. The hormone travels into the cell where it binds to the receptor, triggering actions directly and requiring no intermediary proteins.
SECONDARY MESSENGERS
Some hormones need secondary messengers to cause changes in a cell. A good analogy is the emergency response system. If you phone for an ambulance, the dispatcher does not arrive at your door to take you to hospital. Instead your call triggers a series of actions that brings an ambulance to your home. The dispatcher is the secondary messenger who makes things happen, but it is your call that triggered those actions.
Peptide, protein and some amine hormones need the assistance of secondary messengers since they do not enter the cell themselves. Instead receptors for these hormones protrude from the target cell’s membrane. These hormones are unable to cross the lipid environment of the membrane, which is lipophobic (fat-insoluble). They therefore rely on other proteins inside the cell – the secondary messengers – to bring about changes within it.
Source : Meet Your Hormones: Discover the Hidden World of the Chemical Messengers in Your Body by Catherine Whitlock
Goodreads : https://www.goodreads.com/book/show/44765578-meet-your-hormones
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