Although a given hormone may travel throughout the body in the bloodstream, it will affect the activity only of its target cells; that is, cells with receptors for that particular hormone. Once the hormone binds to the receptor, a chain of events is initiated that leads to the target cell’s response. Hormones play a critical role in the regulation of physiological processes because of the target cell responses they regulate. These responses contribute to human reproduction, growth and development of body tissues, metabolism, fluid, and electrolyte balance, sleep, and many other body functions.

Functions of Hormones

The endocrine system and the nervous system work hand in hand to coordinate the activity of our hormones.  The nervous system consist of neurons, which allow signals to move between the brain and the body.  The endocrine system also plays a vital role in communication throughout our body.  The endocrine system consist of glands located all through the body, which secrete hormones that regulate things such as metabolism. digestion, blood pressure, and growth.  These two systems are not directly linked, however the two interact in many ways.

Hormones are chemical substances that are secreted by cells into extracellular fluids and regulate a variety of activities throughout the body.

Nature of Hormones

The hormones of the human body can be divided into two major groups on the basis of their chemical structure. Hormones derived from amino acids include amines, peptides, and proteins. Hormones can be non steroids( composed of amino acids), steroids(made from cholesterol), and eicosanoids (lipid molecules that cause actions in cells, but are not considered hormones and include prostaglandins and leukotrienes). Damaged tissues release the prostaglandins and leukotrienes to function in vasoconstriction, constriction of respiratory passages, sensitize pain receptors, and inflammation.  These chemical groups affect a hormone’s distribution, the type of receptors it binds to, and other aspects of its function.

Amine Hormones

Hormones derived from the modification of amino acids are referred to as amine hormones. Amine hormones are synthesized from the amino acids tryptophan or tyrosine. An example of a hormone derived from tryptophan is melatonin, which is secreted by the pineal gland and helps regulate circadian rhythm. Tyrosine derivatives include the metabolism-regulating thyroid hormones, as well as the catecholamines, such as epinephrine, norepinephrine, and dopamine. Epinephrine and norepinephrine are secreted by the adrenal medulla and play a role in the fight-or-flight response, whereas dopamine is secreted by the hypothalamus and inhibits the release of certain anterior pituitary hormones.

Peptide and Protein Hormones

Whereas the amine hormones are derived from a single amino acid, peptide and protein hormones consist of multiple amino acids that link to form an amino acid chain. Peptide hormones consist of short chains of amino acids, whereas protein hormones are longer polypeptides. Both types are synthesized like other body proteins: DNA is transcribed into mRNA, which is translated into an amino acid chain.

Examples of peptide hormones include antidiuretic hormone (ADH), a pituitary hormone important in fluid balance, and atrial-natriuretic peptide, which is produced by the heart and helps to decrease blood pressure. Some examples of protein hormones include growth hormone, which is produced by the pituitary gland, and follicle-stimulating hormone (FSH), which has an attached carbohydrate group and is thus classified as a glycoprotein. FSH helps stimulate the maturation of eggs in the ovaries and sperm in the testes.

Steroid Hormones

The primary hormones derived from lipids are steroids. Stroid horns diffuse through the plasma membrane of target cells and enter the nucleus.  They bind to a receptor within the nucleus forming a hormone-receptor complex.  This complex interacts with DNA beginning transcription of specific genes.  mRNA travels to ribosomes where translation takes place.  Steroid hormones are derived from the lipid cholesterol. For example, the reproductive hormones testosterone and the estrogens—which are produced by the gonads (testes and ovaries)—are steroid hormones. The adrenal glands produce the steroid hormone aldosterone, which is involved in osmoregulation, and cortisol, which plays a role in metabolism.

Like cholesterol, steroid hormones are not soluble in water (they are hydrophobic). Because blood is water-based, lipid-derived hormones must travel to their target cell bound to a transport protein. This more complex structure extends the half-life of steroid hormones much longer than that of hormones derived from amino acids. A hormone’s half-life is the time required for half the concentration of the hormone to be degraded. For example, the lipid-derived hormone cortisol has a half-life of approximately 60 to 90 minutes. In contrast, the amino acid–derived hormone epinephrine has a half-life of approximately one minute.

Non steroid Hormone Action

The hormone will bind to a membrane receptor, but the hormone does not enter the cell.  The binding of the first messenger (hormone) catalyzes a reaction that produces a second messenger molecule- usually cyclic adenosine monophosphate (cAMP). The second messenger oversees additional intracellular changes to promote a specific response to the hormone.

Pathways of Hormone Action

The message a hormone sends is received by a hormone receptor, a protein located either inside the cell or within the cell membrane. The receptor will process the message by initiating other signaling events or cellular mechanisms that result in the target cell’s response. Hormone receptors recognize molecules with specific shapes and side groups, and respond only to those hormones that are recognized. The same type of receptor may be located on cells in different body tissues, and trigger somewhat different responses. Thus, the response triggered by a hormone depends not only on the hormone, but also on the target cell.

Once the target cell receives the hormone signal, it can respond in a variety of ways. The response may include the stimulation of protein synthesis, activation or deactivation of enzymes, alteration in the permeability of the cell membrane, altered rates of mitosis and cell growth, and stimulation of the secretion of products. Moreover, a single hormone may be capable of inducing different responses in a given cell.

Factors Affecting Target Cell Response

Target cells/Target organs- are certain tissue cells or organs that a hormone will affect. Target cells must have receptors specific to a given hormone if that hormone is to trigger a response. Target cells must have a specific protein receptor for the hormone to bind or have an effect.

Regulation of Hormone Secretion

To prevent abnormal hormone levels and a potential disease state, hormone levels must be tightly controlled. The body maintains this control by balancing hormone production and degradation. Feedback loops govern the initiation and maintenance of most hormone secretion in response to various stimuli. Hormone levels must maintain homeostasis to prevent disease or disorder.

Role of Feedback Loops

The contribution of feedback loops to homeostasis will only be briefly reviewed here. Positive feedback loops are characterized by the release of additional hormone in response to an original hormone release. The release of oxytocin during childbirth is a positive feedback loop. The initial release of oxytocin begins to signal the uterine muscles to contract, which pushes the fetus toward the cervix, causing it to stretch. This, in turn, signals the pituitary gland to release more oxytocin, causing labor contractions to intensify. The release of oxytocin decreases after the birth of the child.

The more common method of hormone regulation is the negative feedback loop. Negative feedback is characterized by the inhibition of further secretion of a hormone in response to adequate levels of that hormone. This allows blood levels of the hormone to be regulated within a narrow range. An example of a negative feedback loop is the release of glucocorticoid hormones from the adrenal glands, as directed by the hypothalamus and pituitary gland. As glucocorticoid concentrations in the blood rise, the hypothalamus and pituitary gland reduce their signaling to the adrenal glands to prevent additional glucocorticoid secretion (Figure 4).

Figure 4. Negative Feedback Loop

The release of adrenal glucocorticoids is stimulated by the release of hormones from the hypothalamus and pituitary gland. This signaling is inhibited when glucocorticoid levels become elevated by causing negative signals to the pituitary gland and hypothalamus.

Role of Endocrine Gland Stimuli

Reflexes triggered by both chemical and neural stimuli control endocrine activity. These reflexes may be simple, involving only one hormone response, or they may be more complex and involve many hormones, as is the case with the hypothalamic control of various anterior pituitary–controlled hormones. Endocrine gland stimuli consist of humoral, hormonal, and neural.

Humoral stimuli are changes in blood levels of non-hormone chemicals, such as nutrients or ions, which cause the release or inhibition of a hormone to, in turn, maintain homeostasis. For example, osmoreceptors in the hypothalamus detect changes in blood osmolarity (the concentration of solutes in the blood plasma). If blood osmolarity is too high, meaning that the blood is not dilute enough, osmoreceptors signal the hypothalamus to release ADH. The hormone causes the kidneys to reabsorb more water and reduce the volume of urine produced. This reabsorption causes a reduction of the osmolarity of the blood, diluting the blood to the appropriate level. The regulation of blood glucose is another example. High blood glucose levels cause the release of insulin from the pancreas, which increases glucose uptake by cells and liver storage of glucose as glycogen.

Hormones may also be released in response to hormonal stimuli.  An endocrine gland may also secrete a hormone in response to the presence of another hormone produced by a different endocrine gland. Such hormonal stimuli often involve the hypothalamus, which produces releasing and inhibiting hormones that control the secretion of a variety of pituitary hormones. Glands are activated by other hormones which activates another hormone and so forth.

In addition to these chemical signals, hormones can also be released in response to neural stimuli. A common example of neural stimuli is the activation of the fight-or-flight response by the sympathetic nervous system. When an individual perceives danger, sympathetic neurons signal the adrenal glands to secrete norepinephrine and epinephrine. The two hormones dilate blood vessels, increase the heart and respiratory rate, and suppress the digestive and immune systems. These responses boost the body’s transport of oxygen to the brain and muscles, thereby improving the body’s ability to fight or flee.

Glossary

adenylyl cyclase: membrane-bound enzyme that converts ATP to cyclic AMP, creating cAMP, as a result of G-protein activation

cyclic adenosine monophosphate (cAMP): second messenger that, in response to adenylyl cyclase activation, triggers a phosphorylation cascade

diacylglycerol (DAG): molecule that, like cAMP, activates protein kinases, thereby initiating a phosphorylation cascade

downregulation: decrease in the number of hormone receptors, typically in response to chronically excessive levels of a hormone

first messenger: hormone that binds to a cell membrane hormone receptor and triggers activation of a second messenger system

G protein: protein associated with a cell membrane hormone receptor that initiates the next step in a second messenger system upon activation by hormone–receptor binding

hormone receptor: protein within a cell or on the cell membrane that binds a hormone, initiating the target cell response

inositol triphosphate (IP₃): molecule that initiates the release of calcium ions from intracellular stores

phosphodiesterase (PDE): cytosolic enzyme that deactivates and degrades cAMP

phosphorylation cascade: signaling event in which multiple protein kinases phosphorylate the next protein substrate by transferring a phosphate group from ATP to the protein

protein kinase: enzyme that initiates a phosphorylation cascade upon activation

second messenger: molecule that initiates a signaling cascade in response to hormone binding on a cell membrane receptor and activation of a G protein

upregulation: increase in the number of hormone receptors, typically in response to chronically reduced levels of a hormone