The Chemical Signaling

The multiple activities of the cells, tissues, and organs of the body are coordinated by the interplayof several types of chemical messenger systems:

1. Neurotransmitters are released by axon -terminals of neurons into the synaptic junctions and act locally to control nerve cell functions.

2. Endocrine hormones are released by glands or specialized cells into the circulating blood and influence the function of cells at another location in the body.

3. Neuroendocrine hormones are secreted by neurons into the circulating blood and influence the function of cells at another location in the body.

4. Paracrines secreted by cells into the extracellular fluid and affect neighboring cells of a different type.

5. Autocrines are secreted by cells into the extracellular fluid and affect the function of the same cells that produced them by binding to cell surface receptors.

6. Cytokines are peptides secreted by cells into the extracellular fluid and can function as autocrines, paracrines, or endocrine hormones, e.g. interleukins and other lymphokinesthat are secreted by helper cells and act on other cells of the immune system.

 

The Endocrine System

Here, we shall discuss mainly the endocrine hormone system, keeping in mind that many of the body’s chemical messenger systems interact with one another to maintain homeostasis.

The endocrine hormonesare carried by the circulatory system to cells throughout the body, including the nervous system in some cases, where they bind with receptors andinitiate many reactions. Some endocrine hormonesaffect many different types of cells of the body; for example, growth hormonecauses growth in most parts of the body, andthyroxineincreases the rate of many chemical reactions in almost allthe body’s cells. Other hormones affect only specific target tissues, because only these tissues have receptors for the hormone. For example, adrenocorticotropic hormone(ACTH) specifically stimulates the adrenal cortex, causing it to secrete adrenocortical hormones, and the ovarian hormoneshave specific effects on the female sex organs as well as on the secondary sexual characteristics of the female body.

The multiple hormone systems play a key role in regulating almost all body functions, including metabolism, growth and development, water and electrolyte balance, reproduction, and behavior. For instance,without growth hormone, a person would be a dwarf.Without thyroxin and triiodothyronine from the thyroid gland, almost all the chemical reactions of the body would become sluggish, and the person would become sluggish as well.Without insulin from the pancreas, the body’s cells could use little of the food carbohydrates for energy. And without the sex hormones,sexual development and sexual functions would be absent.

 

           

 

Chemical Structure and Synthesis of Hormones

There are three general classes of hormones:

1. Proteins and polypeptides, including hormones secreted by the anterior and posterior pituitary
gland, the pancreas (insulin and glucagon), the parathyroid gland (parathyroid hormone), and
many   others.
2. Steroids secreted by the adrenal cortex (cortisoland aldosterone), the ovaries (estrogen and
progesterone), the testes (testosterone), and the placenta (estrogen and progesterone).
3. Derivatives of the amino acid tyrosine, secreted by the thyroid (thyroxine and triiodothyronine) and the adrenal medullae (epinephrine and norepinephrine). There are no known polysaccharides or nucleic acid hormones.

Polypeptide and Protein Hormones are Stored in Secretory Vesicles Until Needed.

Most of the hormones in the body are polypeptides and proteins. These hormones range in size from small peptides with as few as 3 amino acids (thyrotropin-releasing hormone) to proteins with almost 200 amino acids (growth hormone and prolactin). In general, polypeptides with 100 or more amino acids are called proteins, and those with fewer than 100 amino acids are referred to as peptides. Protein and peptide hormones are synthesized on the rough end of the endoplasmic reticulum of the different endocrine cells, in the same fashion as most other proteins. They are usually synthesized first as larger proteins that are not biologically active (preprohormones) and are cleaved to form smaller prohormones in the endoplasmic reticulum. These are then transferred to the Golgi apparatus for packaging into secretory vesicles. In this process, enzymes in the vesicles cleave the prohormones to produce smaller, biologically active hormones and inactive fragments. The vesicles are stored within the cytoplasm, and many are bound to the cell membrane until their secretion is needed. Secretion of the hormones (as well as the inactive fragments) occurs when the secretory vesicles fuse with the cell membrane and the granular contents are extruded into the interstitial fluid or directly into the blood stream by exocytosis. In many cases, the stimulus for exocytosis is an increase in cytosolic calcium concentration caused by depolarization of the plasma membrane. In other instances, stimulation of an endocrine cell surface receptor causes increased cyclic adenosine monophosphate (cAMP) and subsequently activation of proteinkinases that initiate secretion of the hormone. The peptide hormones are water soluble, allowing them to enter the circulatory system easily, where they are carried to their target tissues.

Steroid Hormones are usually synthesized from Cholesterol and are not stored.

The chemical structure of steroid hormones is similar to that of cholesterol, and in most instances they are synthesized from cholesterol itself. They are lipid soluble and consist of three cyclohexyl rings and one cyclopentyl ring combined into a single structure. Although there is usually very little hormone storage in steroid-producing endocrine cells, large stores of cholesterol esters in cytoplasm vacuoles can be rapidly mobilized for steroid synthesis after a stimulus. Much of the cholesterol in steroid-producing cells comes from the plasma, but there is also de novo synthesis of cholesterol in steroid-producing cells. Because the steroids are highly lipid soluble, once they are synthesized, they simply diffuse across the cell membrane and enter the interstitial fluid and then the blood.

Amine Hormones are derived from Tyrosine

The two groups of hormones derived from tyrosine, the thyroid and the adrenal medullary hormones, are formed by the actions of enzymes in the cytoplasmic compartments of the glandular cells. The thyroid hormones are synthesized and stored in the thyroid gland and incorporated into macromolecules of the protein thyroglobulin, which is stored in large follicles within the thyroid gland. Hormone secretion occurs when the amines are split from thyroglobulin, and the free hormones are then released into the blood stream. After entering the blood, most of the thyroid hormones combine with plasma proteins, especially thyroxine-binding globulin, which slowly releases the hormones to the target tissues. Epinephrine and norepinephrine are formed in the adrenal medulla, which normally secretes about four times more epinephrine than norepinephrine. Catecholamines are taken up into preformed vesicles and
stored until secreted. Similar to the protein hormones stored in secretory granules, catecholamines are also released from adrenal medullary cells by exocytosis. Once the catecholamines enter the circulation, they can exist in the plasma in free form or in conjugation
with other substances.

 

 

 

Hormone Secretion, Transport, and Clearance from the Blood

Hormone Secretion after a Stimulus and duration of Action of different Hormones.

Some hormones, such as norepinephrine and epinephrine, are secreted within seconds after the gland is stimulated, and they may develop full action within another few seconds to minutes; the actions of other hormones, such as thyroxine or growth hormone, may require months for full effect. Thus, each of the different hormones has its own characteristic onset and duration of action—each tailored to perform its specific control function.

Concentrations of Hormones in the Circulating Blood, and Hormonal Secretion Rates.

The concentrations of hormones required to control most metabolic and endocrine functions are incredibly small. Their concentrations in the blood range from as little as 1 picogram in each mL of blood up to at most a few µg (a few millionths of a gram) per mL of blood. Similarly, the rates of secretion of the various hormones are extremely small, usually measured in µg or mg per day. Highly specialized mechanisms are available in the target tissues that allow even these minute quantities of hormones to exert powerful control over the physiological systems.

 

Feedback Control of Hormone Secretion

Negative Feedback Prevents over-activity of Hormone Systems.

Although the plasma concentrations of many hormones fluctuate in response to various stimuli that occur throughout the day, all hormones studied thus far appear to be closely controlled. In most instances, this control is exerted through negative feedback mechanisms that ensure a proper level of hormone activity at the target tissue. After a stimulus causes release of the hormone, conditions or products resulting from the action of hormone tend to suppress its further release. In other words, the hormone (or one of its products) has a negative feedback effect to prevent over-secretion of the hormone or over-activity at the target tissue. The controlled variable is often not the secretory rate of the hormone itself but the degree of activity of the target tissue. Therefore, only when the target tissue activity rises to an appropriate level, will feedback signals to the endocrine gland become powerful enough to slow further secretion of the hormone. Feedback regulation of hormones can occur at all levels, including gene transcription and translation steps involved in the synthesis of hormones and steps involved in processing hormones or releasing stored hormones.

Surges of Hormones can occur with Positive Feedback

In a few instances, positive feedback occurs when the biological action of the hormone causes additional secretion of the hormone. One example of this is the surge of luteinizing hormone (LH) that occurs as a result of the stimulatory effect of estrogen on the anterior pituitary before ovulation. The secreted LH then acts on the ovaries to stimulate additional secretion of estrogen, which in turn causes more secretion of LH. Eventually, LH reaches an appropriate concentration, and typical negative feedback control of hormone secretion is then exerted.

                                               

Cyclical Variations occur in Hormone Release. Superimposed on negative and positive feedback control of hormone secretion are periodic variations in hormone release, influenced by seasonal changes, various stages of development and aging, the diurnal (daily) cycle, and sleep. For example, secretion of growth hormone is markedly increased during early period of sleep but is reduced during the later stages of sleep. Mostly these cyclical variations in hormone secretion are due to changes in activity of neural pathways involved in controlling hormone release.

 

Transport of Hormones in the Blood

Water-soluble hormones (peptides and catecholamines) are dissolved in the plasma and transported from their sites of synthesis to target tissues, where they diffuse out of the capillaries, into the interstitial fluid, and ultimately to target cells.

Steroid and thyroid hormones, in contrast, circulatein the blood mainly bound to plasma proteins. Usuallyless than 10 per cent of steroid or thyroid hormones inthe plasma exist free in solution. For example, more than 99 per cent of the thyroxine in the blood is bound to plasma proteins. However, protein-bound hormones cannot easily diffuse across the capillaries and gain access to their target cells and are therefore biologically inactive until they dissociate from plasma proteins.

The relatively large amounts of hormones bound to proteins serve as reservoirs, replenishing the concentration of free hormones when they are bound to target receptors or lost from the circulation. Binding of hormones to plasma proteins greatly slows their clearance from the plasma.

“Clearance” of Hormones from the Blood

Two factors can increase or decrease the concentration of a hormone in the blood. One of these is the rate of hormone secretion into the blood. Second is the rate of removal of the hormone from the blood, which is called the metabolic clearance rate. This is usually expressed in terms of the number of milliliters of plasma cleared of the hormone per minute.Hormones are “cleared” from the plasma in several ways, including (1) metabolic destruction by thetissues, (2) binding with the tissues, (3) excretion by theliver into the bile, and (4) excretion by the kidneys intothe urine.

Most of the peptide hormones and catecholamines are water soluble and circulate freely in the blood. They are usually degraded by enzymes in the bloodand tissues and rapidly excreted by the kidneys andliver, thus remaining in the blood for only a short time.For example, the half-life of angiotensin II circulatingin the blood is less than a minute.

Hormones that are bound to plasma proteins are cleared from the blood at much slower rates and mayremain in the circulation for several hours or evendays.The half-life of adrenal steroids in the circulation,for example, ranges between 20 and 100 minutes,whereas the half-life of the protein-bound thyroid hormones may be as long as 1 to 6 days.

Mechanisms of Action of Hormones

 

Hormone Receptors and their activation

The first step of a hormone’s action is to bind to specific receptors at the target cell. Cells that lack receptors for the hormones do not respond. Receptors for some hormones are located on the target cellmembrane, whereas other hormone receptors arelocated in the cytoplasm or the nucleus. When the hormone combines with its receptor, this usually initiates a cascade of reactions in the cell, with each stage becoming more powerfully activated so that even small concentrations of the hormone can have a large effect.

Hormonal receptors are large proteins, and each cell that is to be stimulated usually has some 2000 to 100,000 receptors. Also, each receptor is usually highly specific for a single hormone; this determines the type of hormone that will act on a particular tissue. The target tissues that areaffected by a hormone are those that contain its specific receptors.

The locations for the different types of hormone receptors are generally the following:
1. In or on the surface of the cell membrane. The membrane receptors are specific mostly for the
protein, peptide, and catecholamine hormones.

2. In the cell cytoplasm. The primary receptors for the different steroid hormones are found mainly in the cytoplasm.

3. In the cell nucleus. The receptors for the thyroid hormones are found in the nucleus and are
believed to be located in direct association with one or more of the chromosomes.

The Number and Sensitivity of Hormone Receptors are Regulated

The number of receptors in a target cell usually does not remain constant from day to day, or evenfrom minute to minute. The receptor proteins themselves are often inactivated or destroyed during thecourse of their function, and at other times theyare reactivated or new ones are manufactured bythe protein-manufacturing mechanism of the cell.For instance, increased hormone concentration andincreased binding with its target cell receptors sometimes cause the number of active receptors todecrease. This down-regulation of the receptors can occur as a result of (1) inactivation of some of the receptor molecules, (2) inactivation of some of the intracellular protein signaling molecules, (3) temporary sequestration of the receptor to the inside of the cell, away from the site of action of hormones that interact with cell membrane receptors, (4) destruction of the receptors by lysosomes after they are internalized, or (5) decreased production of the receptors. In each case, receptor down-regulation decreases the
target tissue’s responsiveness to the hormone.

Some hormones cause up-regulation of receptors and intracellular signaling proteins; that is, the stimulating hormone induces greater than normal formation of receptor or intracellular signaling molecules by the protein-manufacturing machinery of the target cell, or greater availability of the receptor for interaction with the hormone. When this occurs, the target tissue becomes progressively more sensitive to the stimulating effects of the hormone.

Intracellular Signaling After Hormone Receptor Activation

Almost without exception, a hormone affects its target tissues by first forming a hormone-receptor complex. This alters the function of the receptor itself, and the activated receptor initiates the hormonal effects. To explain this, let us give a few examples of the different
types of interactions.

Ion Channel–Linked Receptors: Virtually all the neurotransmitter substances, such as acetylcholine and norepinephrine, combine with receptors in the postsynaptic membrane. This almost always causes a change in the structure of the receptor, usually opening or closing a channel for one or more ions. Some of these ion channel–linked receptors open (or close) channels for sodium ions, others for potassium ions, others for calcium ions, and so forth. The altered movement of these ions through the channels causes the subsequent effects on the postsynaptic cells. Although a few hormones may exert some of their actions through
activation of ion channel receptors, most hormones that open or close ions channels do this indirectly by coupling with G protein–linked or enzyme-linked receptors, as discussed next.
G Protein–Linked Hormone Receptors. Many hormones activate receptors that indirectly regulate the activity of target proteins (e.g., enzymes or ion channels) by coupling with groups of cell membrane proteins called heterotrimeric GTP-binding proteins (G proteins). There are more than 1000 known G protein–coupled receptors, all of which have seven transmembrane segments that loop in and out of the cell membrane. Some parts of the receptor that protrude into the cell cytoplasm (especially the cytoplasmic tail of the receptor) are coupled to G proteins that
include three (i.e., trimeric) parts—the α, β and γ subunits. When the ligand (hormone) binds to the extracellular part of the receptor, a conformational change occurs in the receptor that activates the G proteins and induces intracellular signals that either (1) open or close cell membrane ion channels or (2) change the activity of an enzyme in the cytoplasm of the cell.
The trimeric G proteins are named for their ability to bind guanosine nucleotides. In their inactive state, α, β and γ subunits of G proteins form a complex that binds guanosine diphosphate (GDP) on the α subunit. When the receptor is activated, it undergoes a conformational change that causes the GDP-bound trimeric G protein to associate with the cytoplasmic part of the receptor and to exchange GDP for guanosine triphosphate (GTP). Displacement of GDP by GTP causes the α subunit to dissociate from the trimeric complex and to associate with other intracellular signaling proteins; these proteins, in turn, alter the activity of ion channels or intracellular enzymes such as adenylyl cyclase or phospholipase C, which alters cell function. The signaling event is rapidly terminated when the hormone is removed and the α subunit inactivates itself by converting its bound GTP to GDP; then the a subunit once again combines with the β and γ subunits to form an inactive, membrane-bound trimeric G protein.

Some hormones are coupled to inhibitory G proteins (denoted Gi proteins), whereas others are
coupled to stimulatory G proteins (denoted Gs proteins). Thus, depending on the coupling of a hormone receptor to an inhibitory or stimulatory G protein, a hormone can either increase or decrease the activity of intracellular enzymes. This complex system of cell membrane G proteins provides a vast array of potential cell responses to different hormones in the various target tissues of the body.

Enzyme-Linked Hormone Receptors. Some receptors, when activated, function directly as enzymes or are closely associated with enzymes that they activate. These enzyme-linked receptors are proteins that pass through the membrane only once, in contrast to the
seven-transmembrane G protein–coupled receptors. Enzyme-linked receptors have their hormone-binding site on the outside of the cell membrane and their catalytic or enzyme-binding site on the inside. When the hormone binds to the extracellular part of the receptor, an enzyme immediately inside the cell membrane is activated (or occasionally inactivated). Although
many enzyme-linked receptors have intrinsic enzyme activity, others rely on enzymes that are closely associated with the receptor to produce changes in cell function. One example of an enzyme-linked receptor is the leptin receptor. Leptin is a hormone secreted by fat cells and has many physiological effects, but it is especially important in regulating appetite and energy balance. The leptin receptor is a member of a large family of cytokine receptors that do not themselves contain enzymatic activity but signal through associated enzymes.

 

Intracellular Hormone Receptors and Activation of Genes.

Several hormones, including adrenal and gonadal steroid hormones, thyroid hormones, retinoid hormones, and vitamin D, bind with protein receptors inside the cell rather than in the cell membrane. Because these hormones are lipid soluble, they readily cross the cell membrane and interact with receptors in the cytoplasm or nucleus. The activated hormone receptor complex then binds with a specific regulatory (promoter) sequence of the DNA called the hormone response element, and in this manner either activates or represses transcription of specific genes and formation of messenger RNA (mRNA). Therefore, minutes, hours, or even days after the hormone has entered the cell, newly formed proteins appear in the cell and become the controllers of new or altered cellular functions.

Many different tissues have identical intracellular hormone receptors, but the genes that the receptors regulate are different in the various tissues. An intracellular receptor can activate a gene response only if the appropriate combination of gene regulatory proteins is present, and many of these regulatory proteins are tissue specific. Thus, the responses of different tissues to a hormone are determined not only by the specificity of the receptors but also by the genes thatthe receptor regulates.

 

Second Messenger Mechanisms for Mediating Intracellular Hormonal Functions

We noted earlier that one of the means by which hormones exert intracellular actions is to stimulate formation of the second messenger cAMP inside the cell membrane. The cAMP then causes subsequent intracellular effects of the hormone. Thus, the only direct effect that the hormone has on the cell is to activate a single type of membrane receptor. The second messenger

does the rest.

Adenylyl Cyclase–cAMP Second Messenger System

Binding of hormones with the receptor allows coupling of the receptor to a G protein. If the G protein stimulates the adenylyl cyclase–cAMP system, it is called a Gs protein, denoting a stimulatory G protein. Stimulation of adenylyl cyclase, a membrane-bound enzyme, by the Gs protein then catalyzes the conversion of a small amount of cytoplasmic ATP into cAMP inside the cell. This then activates cAMP-dependent protein kinase that phosphorylates specific proteins in the cell, triggering biochemical reactions that ultimately leads cell’s response to the hormone. Once cAMP is formed inside the cell, it usually activates a cascade of enzymes. That is, first one enzyme is activated, which activates a second enzyme, which activates a third, and so forth. The importance of this mechanism is that only a few molecules of activated adenylyl cyclase immediately inside the cell membrane can cause many more molecules of the next enzyme to be activated, which can cause still more molecules of the third enzyme to be activated, and so forth. In this way, even the slightest amount of hormone acting on the cell surface can initiate a powerful cascading activating force for the entire cell.

If binding of the hormone to its receptors is coupled to an inhibitory G protein (denoted Giprotein), adenylyl cyclase will be inhibited, reducing the formation of cAMP and ultimately leading to an inhibitory action in the cell. Thus, depending on the coupling of the hormone receptor to an inhibitory or a stimulatory G protein, a hormone can either increase or decrease

the concentration of cAMP and phosphorylation of key proteins inside the cell.

The specific action that occurs in response to increases or decreases of cAMP in each type of target cell depends on the nature of the intracellular machinery— some cells have one set of enzymes, and other cells have other enzymes. Therefore, different functions are elicited in different target cells, such as initiating synthesis of specific intracellular chemicals, causing muscle contraction or relaxation, initiating secretion by the cells, and altering cell permeability.

Thus, a thyroid cell stimulated by cAMP forms the metabolic hormones thyroxine and triiodothyronine, whereas the samecAMP in an adrenocortical cell causes secretion of the adrenocortical steroid hormones. In epithelial cells of the renal tubules, cAMPincreases their permeability to water.

Hormones that act mainly on the Genetic Machinery of the Cell

Steroid Hormones Increase Protein Synthesis

Another means by which hormones act—specifically, the steroid hormones secreted by the adrenal cortex, ovaries, and testes—is to cause synthesis of proteins in the target cells. These proteins then function as enzymes, transport proteins, or structural proteins, which in turn provide other functions of the cells.

The sequence of events in steroid function is essentially the following:

1. The steroid hormone diffuses across the cell membrane and enters the cytoplasm of the cell,

where it binds with a specific receptor protein.

2. The combined receptor protein–hormone then diffuses into or is transported into the nucleus.

3. The combination binds at specific points on the DNA strands in the chromosomes, which activates the transcription process of specific genes to form mRNA.

4. The mRNA diffuses into the cytoplasm, where it promotes the translation process at the ribosomes to form new proteins.

To give an example, aldosterone, one of the hormones secreted by the adrenal cortex, enters the cytoplasm of renal tubular cells, which contain a specific aldosterone receptor protein.Therefore, in these cells,the sequence of events cited earlier ensues. After about 45 minutes, proteins begin to appear in the renal tubular cells and promote sodium reabsorption from the tubules and potassium secretion into the tubules. Thus, the full action of the steroid hormone is characteristically delayed for at least 45 minutes—up to several hours or even days. This is in marked contrast to the almost instantaneous action of some of the peptide and amino acid derived hormones, such as vasopressin and norepinephrine.

Thyroid Hormones Increase Gene Transcription in the Cell Nucleus

The thyroid hormones thyroxineand triiodothyroninecause increased transcription by specific genes in thenucleus. To accomplish this, these hormones first binddirectly with receptor proteins in the nucleus itself;these receptors are probably protein moleculeslocated within the chromosomal complex, andlikely control the function of the genetic promoters oroperators.

Two important features of thyroid hormone function in the nucleus are the following:

1. They activate the genetic mechanisms for the formation of many types of intracellular proteins—probably 100 or more. Many of theseare enzymes that promote enhanced intracellular

metabolic activity in virtually all cells of the body.

2. Once bound to the intranuclear receptors, the thyroid hormones can continue to express their

control functions for days or even weeks.

 

Measurement of Hormone Concentrations in the Blood

Most hormones are present in the blood in extremely minute quantities; some concentrations are as low as one billionth of a milligram (1 picogram) per milliliter. Therefore, it was very difficult to measure these concentrations by the usual chemical means.An extremely sensitive method, however, was developed about 40 years ago that revolutionized the measurement of hormones,

their precursors, and their metabolic end products. This method is called radioimmunoassay.

Enzyme-linked immunosorbent assays (ELISAs) can also be used to measure almost any protein, including hormones. This test combines the specificity of antibodies with the sensitivity of simple enzyme assays.