Hormone Action Part G

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These phosphorylated tyrosines may serve as docking sites for proteins in other signalling pathways. This new vision of how GH functions should lead to the identification of new cellular actions for GH and thereby increase our understanding of how GH regulates growth, differentiation and metabolism. Access provided by.

Hormone receptor

Reprints and Permissions. Advanced search. Skip to main content. Abstract The intracellular pathways by which binding of GH to its receptor elicits its diverse effects on growth, differentiation and metabolism have eluded investigators for many years. Rights and permissions Reprints and Permissions. 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. The primary hormones derived from lipids are steroids. 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.

Types of 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. 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. The message a hormone sends is received by a hormone receptor , a protein located either inside the cell or within the cell membrane. 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. Intracellular hormone receptors are located inside the cell. Hormones that bind to this type of receptor must be able to cross the cell membrane. Steroid hormones are derived from cholesterol and therefore can readily diffuse through the lipid bilayer of the cell membrane to reach the intracellular receptor Figure 2.

Thyroid hormones, which contain benzene rings studded with iodine, are also lipid-soluble and can enter the cell. The location of steroid and thyroid hormone binding differs slightly: a steroid hormone may bind to its receptor within the cytosol or within the nucleus. In contrast, thyroid hormones bind to receptors already bound to DNA.

INTRODUCTION

For both steroid and thyroid hormones, binding of the hormone-receptor complex with DNA triggers transcription of a target gene to mRNA, which moves to the cytosol and directs protein synthesis by ribosomes. Hydrophilic, or water-soluble, hormones are unable to diffuse through the lipid bilayer of the cell membrane and must therefore pass on their message to a receptor located at the surface of the cell.

Except for thyroid hormones, which are lipid-soluble, all amino acid—derived hormones bind to cell membrane receptors that are located, at least in part, on the extracellular surface of the cell membrane. Therefore, they do not directly affect the transcription of target genes, but instead initiate a signaling cascade that is carried out by a molecule called a second messenger. In this case, the hormone is called a first messenger. The second messenger used by most hormones is cyclic adenosine monophosphate cAMP.

In the cAMP second messenger system, a water-soluble hormone binds to its receptor in the cell membrane Step 1 in Figure 3. This receptor is associated with an intracellular component called a G protein , and binding of the hormone activates the G-protein component Step 2. The activated G protein in turn activates an enzyme called adenylyl cyclase , also known as adenylate cyclase Step 3 , which converts adenosine triphosphate ATP to cAMP Step 4. As the second messenger, cAMP activates a type of enzyme called a protein kinase that is present in the cytosol Step 5.

Activated protein kinases initiate a phosphorylation cascade , in which multiple protein kinases phosphorylate add a phosphate group to numerous and various cellular proteins, including other enzymes Step 6. The phosphorylation of cellular proteins can trigger a wide variety of effects, from nutrient metabolism to the synthesis of different hormones and other products. The effects vary according to the type of target cell, the G proteins and kinases involved, and the phosphorylation of proteins. Examples of hormones that use cAMP as a second messenger include calcitonin, which is important for bone construction and regulating blood calcium levels; glucagon, which plays a role in blood glucose levels; and thyroid-stimulating hormone, which causes the release of T 3 and T 4 from the thyroid gland.

Overall, the phosphorylation cascade significantly increases the efficiency, speed, and specificity of the hormonal response, as thousands of signaling events can be initiated simultaneously in response to a very low concentration of hormone in the bloodstream.

The hypothalamic-pituitary axis

However, the duration of the hormone signal is short, as cAMP is quickly deactivated by the enzyme phosphodiesterase PDE , which is located in the cytosol. Importantly, there are also G proteins that decrease the levels of cAMP in the cell in response to hormone binding. For example, when growth hormone—inhibiting hormone GHIH , also known as somatostatin, binds to its receptors in the pituitary gland, the level of cAMP decreases, thereby inhibiting the secretion of human growth hormone.

Mechanism of Hormone Action -Endocrine System - Bhushan Science

Not all water-soluble hormones initiate the cAMP second messenger system. One common alternative system uses calcium ions as a second messenger. In this system, G proteins activate the enzyme phospholipase C PLC , which functions similarly to adenylyl cyclase. At the same time, IP 3 causes calcium ions to be released from storage sites within the cytosol, such as from within the smooth endoplasmic reticulum. The calcium ions then act as second messengers in two ways: they can influence enzymatic and other cellular activities directly, or they can bind to calcium-binding proteins, the most common of which is calmodulin.


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Upon binding calcium, calmodulin is able to modulate protein kinase within the cell. Examples of hormones that use calcium ions as a second messenger system include angiotensin II, which helps regulate blood pressure through vasoconstriction, and growth hormone—releasing hormone GHRH , which causes the pituitary gland to release growth hormones.

You will recall that target cells must have receptors specific to a given hormone if that hormone is to trigger a response. Examples of hormones that use cAMP as a second messenger include calcitonin, which is important for bone construction and regulating blood calcium levels; glucagon, which plays a role in blood glucose levels; and thyroid-stimulating hormone, which causes the release of T 3 and T 4 from the thyroid gland. Overall, the phosphorylation cascade significantly increases the efficiency, speed, and specificity of the hormonal response, as thousands of signaling events can be initiated simultaneously in response to a very low concentration of hormone in the bloodstream.

However, the duration of the hormone signal is short, as cAMP is quickly deactivated by the enzyme phosphodiesterase PDE , which is located in the cytosol.

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hormones in animal production

Importantly, there are also G proteins that decrease the levels of cAMP in the cell in response to hormone binding. For example, when growth hormone—inhibiting hormone GHIH , also known as somatostatin, binds to its receptors in the pituitary gland, the level of cAMP decreases, thereby inhibiting the secretion of human growth hormone. Not all water-soluble hormones initiate the cAMP second messenger system. One common alternative system uses calcium ions as a second messenger. In this system, G proteins activate the enzyme phospholipase C PLC , which functions similarly to adenylyl cyclase.

At the same time, IP 3 causes calcium ions to be released from storage sites within the cytosol, such as from within the smooth endoplasmic reticulum. The calcium ions then act as second messengers in two ways: they can influence enzymatic and other cellular activities directly, or they can bind to calcium-binding proteins, the most common of which is calmodulin. Upon binding calcium, calmodulin is able to modulate protein kinase within the cell. Examples of hormones that use calcium ions as a second messenger system include angiotensin II, which helps regulate blood pressure through vasoconstriction, and growth hormone—releasing hormone GHRH , which causes the pituitary gland to release growth hormones.

You will recall that target cells must have receptors specific to a given hormone if that hormone is to trigger a response. But several other factors influence the target cell response. For example, the presence of a significant level of a hormone circulating in the bloodstream can cause its target cells to decrease their number of receptors for that hormone. This process is called downregulation , and it allows cells to become less reactive to the excessive hormone levels. When the level of a hormone is chronically reduced, target cells engage in upregulation to increase their number of receptors.

This process allows cells to be more sensitive to the hormone that is present. Cells can also alter the sensitivity of the receptors themselves to various hormones. Two or more hormones can interact to affect the response of cells in a variety of ways. The three most common types of interaction are as follows:. 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. 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.

Reflexes triggered by both chemical and neural stimuli control endocrine activity.