Some consumers view the word hormone with concern. What is a hormone? In this Blog I will discuss what hormones are, what the endocrine system is, and that it is a natural part of biology.
The word hormone is derived from the Greek verb hormao, meaning to excite or arouse. A hormone is a substance that sets in motion a set of metabolic events that would otherwise lie dormant. All of the hormones together form a communication network in the body that is called the endocrine system. Another way of viewing the endocrine system is to imagine at any given moment the circulatory system (the blood) of animals and humans is literally packed with thousands of these chemical messengers moving about the body sort of like an urban freeway on a late Friday afternoon.
Without hormones and the endocrine system, humans and animals would not survive. It is important to appreciate, however, that there are many human diseases which are endocrine in nature. For example diabetes is one such disease, and a standard treatment for many is to receive recombinant human insulin by injection. Moreover, many diseases and conditions result from a “problem” with the endocrine system. Dwarfism is caused by an reduced ability of the body to produce growth hormone (GH)/somatotropin (ST). Cushing’s syndrome is a hormonal disorder caused by prolonged exposure of the body tissues to high levels of the steroid hormone cortisol. The abuse of anabolic steroid hormones is another example of what can occur when excessive use occurs.
The Endocrine System
The endocrine system is a control system of ductless glands, organs and tissues in humans and animals that secrete chemical messengers (i.e., the hormones) that circulate within the body via the bloodstream to affect distant cells. Hormones act as “messengers,” and are carried by the bloodstream to different cells in the body, which interpret these messages and act on them. Typical endocrine glands are pituitary, thyroid, and adrenal glands, but not exocrine glands such as salivary glands, sweat glands and glands within the gastrointestinal tract.
The endocrine system is comprised primarily of two different classes of hormones: proteins and steroid hormones. Protein hormones are made of individual amino acids linked together. Protein hormones are made using the 20 naturally occurring amino acids. The sequence of each hormone is unique and is specified by the genetic code stored in the gene (as DNA) in the nucleus. Protein hormones come in all sizes and shapes. Steroid hormones are lipids and are synthesized from cholesterol.
There are literally hundreds of hormones circulating in the blood at any point in time. The vast majority of these hormones are present at concentrations less that 10 nanograms (ng)/ml (1 ng = 1 x 10-9 grams) of blood. The concentration of many hormones in the blood fluctuates markedly during the day, and many hormones fluctuate monthly and seasonally.
A critical aspect of the endocrine system is that for a hormone to have any biological effect it must be recognized by target cells in the body. This is achieved by binding to receptors on target cells where they initiate a vital cascade of life-sustaining events inside each target cell. A cell that can respond to a specific hormone is called a target cell for that particular hormone. There is great specificity in the endocrine system. Not all cells respond to all hormones; thus, certain hormones may have very potent effects in some cells and no effect in other cell types. This is the basis for bovine ST (bST) not having biological effects in humans – it is not recognized (bound) by human cells. Thus, no biological events take place in the cell.
In animals and other higher organisms, hormones originate in the specific regions of the body, unique places where the cells have become specialized, each to perform a single function. These hormone-manufacturing glands act together, constantly communicating with each other by feedback signals to achieve an optimal metabolic balance throughout the body. The major hormone-producing glands of the body are shown in Table 1. Table 1 also presents a listing of the products (i.e., the hormones) made by these endocrine glands and some of their functions. The hormones produced are secreted directly into the blood.
The hypothalamus is a region of the brain located at the base of the brain. There are at least seven different hypothalamic hormones which are made in the hypothalamus (Table 2). After synthesis, these hormones are released directly into a specialized network of blood vessels called the hypothalamic-pituitary portal circulation. This portal circulation carries these protein hormones to the pituitary gland where they stimulate or inhibit the release of pituitary hormones.
The anterior pituitary gland is the size of a pea and is attached to the base of the brain by a short stalk. The hormones made in the pituitary gland and secreted are shown in Table 1. With respect to regulating animal growth the anterior gland is the master gland, and growth hormone is the master hormone that regulates growth.
a. Are growth hormone and somatotropin (ST) the same hormones?
Somatotropin and growth hormone are different names for the same protein hormone. This hormone contains 191 amino acids and plays an important role in regulating animal growth, lactation, and metabolism.
The first great discovery in endocrinology was the discovery of hormones and the role they played in regulating growth and metabolism. Between the introduction of the word hormone in 1905 and the 1960s it became clear that without hormones life would not exist. One of the great unknown questions was: How do hormones affect cell metabolism and growth? Evidence accumulated to show that hormones acted specifically; that is, they affected some cells but not others.
It became clear that some mechanism had to exist that allowed a cell to specifically recognize a hormone and then to couple this recognition event to some intracellular signal system that changed cell function. Subsequent research has shown that cells respond to protein hormones by a process that involves the hormone receptor binding the hormone. This is a highly specific recognition mechanism. For example, the receptor for insulin does not recognize nor bind growth hormone/somatotropin. By analogy, it is helpful to think of the interaction between a protein hormone and its receptor as being similar to a lock and key; only one key can fit into a specific lock. If the key doesn’t fit the lock it does not open. From an endocrine perspective, if the hormone receptor cannot recognize the hormone then no biological function occurs. In humans, there is a rare disease called Laron Dwarfism in which the cells cannot recognize circulating growth hormone. As a result, the child does not grow and is not responsive to the standard clinical treatment which involves administration of recombinant human GH (hGH/hST).
One good example of hormone specificity relates to hGH (or hST) and bGH (or bST). There is convincing evidence that non-primate GH/ST does not bind to human cells. This means that bGH (bST) does not bind to human cells because the human GH receptor does not recognize it. Consequently, no biological action can occur in response to bST administration in humans. Moreover, since bST is a protein hormone, it is digested (broken down) to individual amino acids in the stomach and intestine if it were consumed. There is absolutely no hormone-like activity that occurs as the result of amino acids being absorbed.
It is interesting how certain words get interpreted in ways that can be concerning to some. Hormones are an essential and natural part of the biology of life. Without them animals would not survive. The endocrine system plays a very important role in allowing cells and tissues to communicate with each other. Moreover many remarkable medical and agricultural biotechnologies that benefit society are based on strategies that either deliver exogenous hormones or affect the production and biological actions of hormones circulating in the blood.