Mechanism of Hormone Action and Regulation

There are specific receptors for the binding of hormones. Steroid hormones have nuclear receptors. These steroid hormones are composed of cholesterol a dare hydrophobic in nature. They enter the cells through cell membranes. They move towards the nucleus and bind to the respective receptor. They have the activity like repression, initiation or transcription of certain genes and products. 

•  Eicosanoids are compounds that include prostaglandins, prostacyclins, leukotrienes, and thromboxanes.
• Lipid insoluble hormones such as eicosanoids bind to receptors on the outer surface of the plasma membrane, via plasma membrane hormone receptors.
  
• The amino acid-derived hormones epinephrine and norepinephrine bind to beta-adrenergic receptors on the plasma membrane of cells.
  
• Hence the Epinephrine and eicosanoids find their receptors on the plasma membrane.
• So, the correct answer is 'Plasma membrane'.

Hormones are generally cell-specific because target cells possess specific receptors that bind only to particular hormones. Protein hormones interact with membrane-bound extracellular receptors, iodothyronines act via intracellular receptors, and somatostatin is secreted by tissues outside the hypothalamus as well.

Intracellular receptors, which bind to lipid-soluble hormones like steroids, are primarily located in the cytoplasm or the nucleus.

Steroid hormones, i.e., androgens, estrogens, glucocorticoids, mineralocorticoids, and progestins, bind with high affinity to their respective steroid hormone receptors (SR).

When hormones bind to membrane receptors, they often trigger the production of second messengers such as cAMP, cGMP, or IP3 (inositol trisphosphate).

Elevated levels of cAMP act as a second messenger that activates protein kinase A, which then phosphorylates target proteins to produce a cellular response.

The ions which plays major role in generating hormone action is calcium ions. They make bones strong and powerful.

cAMP, DAG, and IP3 are well-known second messengers. Phospholipase C is an enzyme that generates second messengers (like DAG and IP3), but it is not a second messenger itself.

• Membrane bound receptors : These are present on the cell membrane of target cells.
• Intracellular receptors: These are present inside the target cell.

• Membrane bound receptors : These are present on the cell membrane of target cells.
• Intracellular receptors: These are present inside the target cell.

Hormones are of 2 types based on their chemical composition. 

Steroid hormones are made up of fats and they are not soluble in water. They are hydro phobic in nature. Hence they can easily pass through the plasma membrane by simple diffusion. 

Cortisol and testosterone are lipid soluble hormones, which can directly pass through the cell membrane of the target cell and bind with intracellular receptors.
So, the correct answer is option D.

Secondary messengers are intracellular signalling molecules released by the cell in response to exposure of extracellular signalling molecules-the first messengers. Second messengers trigger physiological changes such as proliferation, migration, survival, etc. Some common secondary messengers are cAMP(Cyclic Adenosine Mono Phosphate), cGMP(Cyclic Guanosine Mono Phosphate), Calcium ion, IP$ _3$, etc. So all the given options serve as secondary messengers except the Sodium/Mg ions.

A. Thyroxine, insulin hormones are protein hormones. 

In the human body, every receptor is specific for a hormone and every hormone is specific for a receptor.

So, the correct option is ‘Both
true’.

Thyroxine is a proteinaceous hormone, formed by the addition of iodine to amino acid tyrosine which is then combined with protein thyroglobulin

Peptide hormones are hydrophilic due to which they are unable to cross the lipid plasma membrane. They bind to the receptors on the plasma membrane and affect the activity of proteins inside the cell called secondary messenger. These secondary messengers then perform the action required. The steroid hormones are hydrophobic, they can cross the plasma membrane directly and are able to affect the synthesis of proteins directly in the cells.

Prolactin release inhibiting hormone is an amino acid derivative hormone produced in the hypothalamus.

Cyclic adenosine monophosphate is a secondary messenger important in many biological processes. It is a derivative of ATP. When a G-protein–coupled receptor on the surface of a cell binds to its particular ligand, the receptor may stimulate the intracellular production of cAMP by the adenylyl cyclase-associated with that receptor. cAMP can then diffuse through the cytoplasm, where it interacts with various targets. Thus, cAMP serves as a second messenger within a cell.

Steroid hormones, being hydrophobic molecules, diffuse freely into all cells and act within the cell. Steroid hormones secreted by adrenal cortex, ovaries, and testes do not utilize the second messenger to exert their influence. Steroid hormones enter the cytoplasm of a target cell and bind with specific receptor proteins (mobile). Hormone-receptor complex than diffuses into the nucleus and activates specific genes to form a new protein. This protein carries out the specific response for a particular steroid hormone.

Lipid insoluble hormones such as epinephrine acts or functions on a particular site by binding to it. These hormones have their receptors on the plasma membrane of the target cells. The binding of hormone to the receptor initiates a series of events. These series of events are called as signal transduction. In this, the hormone first binds to the surface receptor. It leads to the activation of the enzyme adenylate cyclase. This enzyme converts the ATP into cyclic AMP or cAMP inside the cell. This cAMP acts as second messenger. This cAMP then activates cAMP-dependent protein kinase called protein kinase A. This proteni kinase A triggers a series of molecular interactions that leads to glycogenolysis. This results in the synthesize of glucose from glycogen.

Steroid hormone receptors are found on the plasma membrane, in the cytosol and also in the nucleus of target cells. They are generally intracellular receptors (cytoplasmic) and initiate signal transduction for steroid hormones, which lead to changes in gene expression over a time period of hours to days. 

Though hormones are released into the blood stream and hence can reach all cells, yet they affect only those cells which have hormone specific receptors. Thus, binding of hormone to receptor is crucial irrespective of presence of receptors inside the cell or on its surface. This makes options A and D incorrect as absence of receptors renders the cell unaffected by hormonal action. Option B is incorrect as hormone-receptor binding is essential step in hormonal action. Thus, in absence of receptors, the target organ will no longer respond to hormone. Option C is correct. 

Steroid hormones are a group of hormones, that belong to the class of chemical compounds known as steroids; they are secreted by three steroid glands, the adrenal cortex, testes and ovaries, during pregnancy by the placenta. All steroid hormones are derived from cholesterol. They are transported through the bloodstream to the cells of various target organs, where they carry out the regulation of a wide range of physiological functions.

Though hormones are released into the blood stream and hence can reach all cells, yet they affect only those cells which have hormone specific receptors. Thus, binding of hormone to receptor is crucial irrespective of presence of receptors inside the cell or on its surface. Thus, options A and D are incorrect as the absence of receptors renders the cell unaffected by hormonal action. Option B is incorrect as hormone-receptor binding is an essential step in hormonal action. Thus, in absence of receptors, the target organ will no longer respond to the hormone. Hence, the correct answer is option C.

• A tropic hormone is a hormone that stimulates an endocrine gland to grow and secrete its hormones. (One hormone causes another hormone to do something) 
• In humans, tropic hormones are secreted by the adenohypophysis(anterior pituitary gland). Hence, A hormone that stimulates an endocrine gland to grow and secrete its hormone is known as Tropic hormones

Receptor is a molecule usually found on the surface of a cell, that receives chemical signals from outside the cell. When such external substances bind to a receptor, they direct the cell to do something, such as divide, die, or allow specific substances to enter or exit the cell.

Receptors are proteins embedded in either the cell's plasma membrane (cell surface receptors), in the cytoplasm, or in the cell's nucleus (nuclear receptors), to which specific signaling molecules may attach.

Glucagon is a peptide hormone that binds to cell-surface receptors and utilizes cAMP as a second messenger. Estrogen, cortisol, and iodothyronines are lipid-soluble and act via intracellular receptors.

• Parathyroid hormone (PTH) is essential for the physiologic maintenance of mineral homeostasis. 
• PTH regulates the mineral transport in bone and kidney and through its secondary actions on mineral transport in the intestine. 
• Calcitonin, in many ways, acts as a physiologic antagonist to PTH. Hence,  Parathyroid hormone is antagonistic to calcitonin

Hormones are the chemical substance, released from endocrine glands. They are directly released into the blood and affect the targeted organ. They may be the peptide such as insulin or steroid such as cortisol. Thus, the correct answer is option B.

Hormonal actions are regulated by two mechanisms: feedback and antagonistic effects of two hormones. A feedback mechanism can work as a negative or positive mechanism. The negative feedback mechanism is activated by surplus and proceeds to correct the process that caused the surplus. Therefore, increased the blood level of a certain hormone or its effect on target organ act to prevent its further release. For example, increased blood glucose level stimulates insulin secretion. The positive feedback mechanism of hormone regulation includes stimulation of release of the same hormone caused by its increased blood level. For example, the release of oxytocin is stimulated by uterine contraction during labor pain. 

Gonadotropin is a hormone which stimulates production of sex hormones and development of secondary sexual characters.
Females : Ovaries : Oestrogen and Progesterone
Males : Testes (testicle) : Testosterone.

When the secretion of hormone is controlled by the factors or other hormones it is called feedback control. For example, homeostasis means keeping the internal environment of the body constant. Hormones help in maintaining internal environment of the body. Thus, the correct answer is option B.

An increased amount of thyroxine in the blood exerts an inhibitory effect on the hypothalamus. in such a way that less of thyroid releasing hormone (TRH) and thyroid stimulating hormone (TSH) is produced respectively. This eventually results in a decrease in thyroxine. This is called negative feedback control. 

A- Levels of parathyroid hormone are regulated by a negative feedback loop by increasing blood calcium levels which further prevents the release of parathyroid hormone.

When a high level of particular hormone inhibits their further secretion is called as negative feedback. As the level of water in the blood falls, negative feedback ensures that the amount of ADH rises. As the level of water in the blood rises, negative feedback ensures that the amount of ADH falls. Thus, the correct answer is option B.

When the secretion of hormone is controlled by some factors or other hormones is called as feedback control. It can be both positive and negative. When the hormone secretion is amplified by its own secretion is called as positive feedback. When a high level of particular hormone inhibits their further secretion is called as negative feedback. Thus, the correct answer is option C.

• A feedback mechanism for multistep reactions is a biological occurrence wherein the output of a system amplifies the system (positive feedback) or inhibits the system (negative feedback).
• For example, Release of estrogen from the ovaries stimulates the secretion of GnRH from Hypothalamus which in turn stimulates the secretion of LH from anterior pituitary.when the levels of LH reaches above the normal levels in the blood it stimulates the release of progesterone from corpus luteum which causes the inhibition of the release of GnRH from the hypothalamus which in turn leads to inhibition of release of LH(product) from anterior pituitary and prevents the accumulation of LH(product)in blood. Hence negative feedback.
  
• Hence Negative feedback occurs when The rate of process decreases as the concentration of product increases.
• So, the correct answer is 'The rate of process decreases as the concentration of product increases'.

Hemostasis is the process of blood coagulation or the formation of a clot to prevent blood loss when there is any injury. The main component of hemostasis is based on the positive feedback mechanism. There is activation of one coagulation factor which activates the other coagulation factor. This initiates a cascade mechanism which results in the formation of a proper clot. The mechanism stops or gets inhibited when there is formation of clot.  

• A feedback mechanism for multistep reactions is a biological occurrence wherein the output of a system amplifies the system (positive feedback) or inhibits the system (negative feedback).
• For example, Release of estrogen from the ovaries stimulates the secretion of GnRH from Hypothalamus which in turn stimulates the secretion of LH from anterior pituitary.when the levels of LH reaches above the normal levels in the blood it stimulates the release of progesterone from corpus luteum which causes the inhibition of the release of GnRH from the hypothalamus which in turn leads to inhibition of release of LH(product) from anterior pituitary and prevents the accumulation of LH(product)in blood. Hence negative feed back.
• Hence Negative feedback inhibition prevents the accumulation of product.
• So, the correct answer is 'Negative feedback inhibition'.

• A feedback mechanism for multistep reactions is a biological occurrence wherein the output of a system amplifies the system (positive feedback) or inhibits the system (negative feedback).
• For example, Release of estrogen(input) from the ovaries stimulates the secretion of GnRH from Hypothalamus which in turn stimulates the secretion of LH(product) from the anterior pituitary. This LH further causes the release of more estrogen(input) from the ovaries. Hence increase in input . Thus positive feedback.
• Hence  Positive feedback inhibition is concerned about Increase in the input.
  
• So, the correct answer is 'Increase in input'.

Positive feedback occurs when a system's output enhances or amplifies the process that created it. In gene regulation, this often involves complex feedback loops where a gene product directly or indirectly promotes its own further expression.

Most of the hormones, especially the hormones of the pituitary gland are controlled by the negative feedback inhibition. This system operates between endocrine gland secreting the hormone and the target cells. For example ACTH secreted by the anterior lobe of pituitary stimulates the target cells present in the adrenal gland to produce cortisol. As soon as the concentration of cortisol rises above the set point in the blood stream it acts on pituitary gland to inhibit the secretion of ACTH. Thus option A is the correct answer.

• A feedback mechanism for multistep reactions is a biological occurrence wherein the output of a system amplifies the system (positive feedback) or inhibits the system (negative feedback).
• For example, a positive feedback system is childbirth. During labour, a hormone called oxytocin is released that intensifies and speeds up contractions and facilitates childbirth.
• Hence A major regulatory mechanism for multistep reactions is the feedback mechanism.
• So, the correct answer is 'Feed-back mechanism'.

• The prostaglandins are a group of physiologically active lipid compounds called eicosanoids having diverse hormone-like effects in animals.
• Prostaglandins have been found in almost every tissue in humans and other animals. 
• They are derived enzymatically from the fatty acid arachidonic acid. Every prostaglandin contains 20 carbon atoms, including a 5-carbon ring. 
• They are a subclass of eicosanoids and of the prostanoid class of fatty acid derivatives. Hence, Prostaglandins are complex fatty acids that function as intercellular messengers.

Negative feedback, a self-regulated process occurs when some function of the output of a system process, or mechanism is fed back in a manner that tends to reduce the fluctuation in the output whether caused by the changes in the input or by other disturbance.

Endocrine glands----> hormones-----> blood----> organs / tissues----> biochemical / physiological activity.

In feed back inhibition product of a reaction inhibits the enzyme catalysing that reaction. It is a type of control mechanism at the enzyme level. If the product is produced in sufficient amount, it inhibits the enzyme to stop the further production. So, the correct answer is 'End product'.

Accumulation of end products triggers an inhibiting action as a feedback inhibition of the metabolic pathway causing its inhibition. as a result, the metabolic pathway is switched off or halted. For example, the absence of the suckling stimulus leads to inhibition of the milk secretion.

Most hormones are regulated by feedback mechanisms. A feedback mechanism is a loop in which a product feeds back to control its own productions. Most negative feedback loops get involved from hormone feedback mechanisms. Here the time delay occurs and hormones are released.

There are two sets of nerve cells in the hypothalamus that produce hormones.

The feedback mechanism is the process wherein the upregulation and downregulation of genes, hormones, homoeostasis and other factors are controlled. For example, if the level of sugar concentration in blood has increased feedback mechanism upregulates the hormone insulin which helps to stabilise the sugar in the blood. 

One distinctive feature of hormones whose secretion is regulated through the hypothalamus and pituitary is that they regulate their own secretion through negative feedback inhibition. CRH (corticotropin releasing hormone) and ACTH (adrenocorticotropic hormone). Less tropic hormone secretion leads to less stimulation of cortisol secretion by cells of the zona fasciculata of the adrenal cortex. The usefulness of negative feedback inhibition is that it works to keep hormone levels within a particular appropriate physiological range.  The reduced negative feedback inhibition means that more CRH and ACTH will be secreted. More ACTH will stimulate the remaining adrenal tissue to grow and to secrete more cortisol. This will have the effect of bringing cortisol back up towards its normal daily level of secretion.

The hypothalamus and pituitary that triggered the female reproductive cycle do not schedule milk production on a nine-month clock basis. But oxytocin is released due to the stimulation by fetal ejection reflex. By this stimulation, oxytocin is released which causes contraction in uterus wall and also stimulates prolactin secretion for milk production in mammary gland. Thus, the correct answer is option D.

Allosteric inhibition is the regulation of an enzyme by an effector molecule that binds to a site other than the active site, which is called allosteric site. Since it is not blocking the active site it is called 'positive feedback' option $(B)$

A positive feedback loop involving cytokines leads to an excessive and uncontrolled release of cytokines, a condition known as a cytokine storm or hypercytokinemia. Hypocytokinemia refers to a deficiency of cytokines, which is the opposite of this effect.

• Negative feedback is a reaction that causes a decrease in function. It occurs in response to some kind of stimulus. Often it causes the output of a system to be lessened; so, the feedback tends to stabilize the system. This can be referred to as homeostasis. For example, constriction of skin blood vessels and contraction of skeletal muscles in cold.
• So, the correct answer is 'Constriction of skin blood vessels and contraction of skeletal muscles in cold'.