Hormones: How do they work in the body PART I

Over the past months I have discussed how muscle cells function and metabolize. Now I would like to discuss the endocrine system with respect to maintenance of homeostasis during both resting and exercising conditions. Hormones are what all physiological reactions are based on in the human body. To understand how the muscle functions it is important to know how hormones secrete and produce reactions within the systems of the body. The impact of training on circulating blood levels of selected hormones will also be considered.

Characteristics of Hormone Action

A hormone is defined as a discrete chemical substance that is secreted into the body fluids by an endocrine gland and that has a specific effect on the activities of other cells, tissues and organs. The cell, tissue, or gland on which a hormone has an effect is called a target cell, or target tissue, or target organ, respectively.

The endocrine glands are ductless and are composed of epithelial cells in which hormones are manufactured or stored. Because the hormone is secreted directly into the blood or lymph, the endocrine glands are referred to as glands of internal secretion

As just mentioned, hormones cause a specific effect on the activities of target organs. This effect, which may require minutes or hours to occur, is brought about mainly by increasing or decreasing an ongoing cellular process rather than by initiating a new one. For example, hormones may (1) activate enzyme systems, (2) alter cell membrane permeability, (3) cause muscular contraction or relaxation, (4) stimulate protein synthesis, or (5) cause cellular secretion.

Three general characteristics of hormone action that need to be discussed are (1) specificity of hormone action, (2) physiological mechanisms of hormone action, and (3) control of hormone secretion.

Specificity of Hormone Action

Although some hormones have an effect on all tissues of the body, most have an effect only on a specific target organ. This specificity is accomplished by the presence of a specific hormone receptor located within the cell membrane of the target organ. The receptor is specific to and can react with only one hormone. It is analogous to a lock and key; only a specific key (hormone) will fit the lock (receptor), thus opening the way for a given action. Hormones may be so specific that they affect only a specific part of an organ or tissue. For example, antidiuretic hormone (ADH) affects cells of the collecting tubules in the kidney, but not those of the ascending limb of the loop of Henle.

It is thought that hormones that cause an effect on all the tissues of the body also work by the receptor mechanism. However, in this case, the receptor is more general and widespread so that all the cells have them. Examples are the receptors for thyroxin and growth hormone.

Mechanism of Hormone Action

Physiologically , how does a hormone cause an effect on a cell? There are many different physiological mechanisms of hormonal action. However, the most common mechanism of action of the majority of hormones is the cyclic AMP mechanism. AMP is an abbreviation for adenosine monophosphate, a compound similar to ATP (adenosine troposphere). Because it is involved in the mechanism of action of so many hormones, it is often referred to as a messenger for hormone mediation.

The cyclic AMP mechanism of hormone action. A hormone, on reaching the cell via the blood, interacts with its specific receptor located within the cell membrane. This interaction activates an enzyme called adenyl cyclase, which is also located within the cell membrane. In turn, the activated adenyl cyclase causes cyclic AMP to be formed into ATP, which is located inside the cell in the cytoplasm. Once cyclic AMP is formed, one or more of the physiological responses mentioned before can occur. The response ceases when cyclic AMP is destroyed. The particular response that occurs depends on the type of cell itself. For example thyroid cells stimulated by cyclic AMP form thyroid hormone, whereas epithelial cells of the renal tubules are affected by cyclic AMP by increasing their permeability to water. Also, several hormones may cause the same response in a given cell. Fat cells, for instance, can be stimulated through the cyclic AMP mechanism to break down triglycerides by the hormones epinephrine, nor epinephrine, adrenocorticotropic hormone, and glucagon

It is thought that cyclic AMP is not the only type of intracellular hormone mediator. Other such substances might include. (1) prostaglandins, a series of lipid compounds present in most cells throughout the body; and (2) a compound called cyclic guanosine monophosphate, which is similar to cyclic AMP. In addition, the intracellular hormone mediator mechanism is not the only mechanism whereby hormones can elicit a cellular effect. For example, insulin causes a direct effect on the permeability of cell membranes to glucose, whereas the catechololamine hormones cause a direct effect on membrane permeability to various ions.

Control of Hormone Secretion

Because hormones have a precise effect on cellular function, their secretion must also be precisely controlled. How is this accomplished? Again, several control systems exist. One of these is the nervous system. However, the predominant hormonal control system is the negative feedback mechanism. Basically, in the mechanism, the secretion of the hormone is turned off or decreased due to the end result of the response caused by that hormone.

An increase in blood glucose concentration stimulates the pancreas to secrete the hormone insulin. Insulin causes an increase in cellular glucose uptake, which decreases the blood glucose concentration. This decrease in blood glucose “feed back” to the pancreas, having a “negative” (decreased) effect on the secretion of insulin (hence, the term negative feedback). In other words, the end result of the action of insulin (decreased blood glucose) causes its secretion to be turned off or reduced.

Although this is a relatively simple example, some hormones are controlled by a more complex version of the negative feedback mechanism. The secretion of Thyroxin from the thyroid gland, for instance, is stimulated by another hormone called thyroid-stimulating hormone, or TSH, from the anterior pituitary gland. The negative feedback in this case is provided by the level of thyroxin in the blood. When it is high, secretion of TSH is reduced; when it is low, TSH secretion is increased. In still other feedback systems, several endrocrine glands and their hormones might be involved.

As mentioned previously, the nervous system is also involved in the control of hormone secretion. For example, epinephrine and norepinephrine from the adrenal medulla are secreted in direct response to stimulation by the sympathetic nervous system. The release of ant diuretic hormone (ADH) from the posterior pituitary gland is also under control from the brain. Actually, the control of hormone secretion by the nervous system in not surprising. These two systems must, and do, work together to bring about the precise regulation necessary for maintenance of homeostatic function.

Daryl Conant, M.Ed