Hormones

Hormone:

A chemical substance synthesized in small amounts by an endocrine tissue and carried in the blood to another tissue, where it acts as a messenger to regulate the function of the

target tissue or organ.

-Hormones control not only different aspects of metabolism but also many other functions (e.g., cell and tissue growth, secretion of digestive enzymes, the reproductive system)

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●The hypothalamus, a specialized portion of the brain, is the

coordination centre of the endocrine system.

-It receives and integrates messages from the central nervous

system. in response to these messages the hypothalamus

produces a number of regulatory hormones, which are sent to the

anterior pituitary gland, located just below the hypothalamus.

-Some hypothalamic hormones (releasing factors) stimulate the

anterior pituitary to secrete a given hormone; others inhibitory.

Hormones function in a complex interrelated hierarchy:

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●Once it is stimulated, the anterior pituitary secretes hormones into the blood to be carried to the next rank of endocrine glands, which includes the adrenal cortex, the thyroid gland, endocrine cells of the pancreas.

-These glands in turn are stimulated to secrete their specific hormones, which are carried by the blood to ultimate target tissue.

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• Hypothalamus.

• Pituitary gland.

• Thyroid.

• Parathyroids.

• Adrenal glands.

• Pineal body.

• Pancreas.

• Reproductive glands (which include the ovaries and testes)

The major endocrine glands: The major glands that make up the human endocrine system

include the:

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Hormones secreted by different endocrine glands vary widely in chemical structure.

Hormones can be divided into:

1) proteins:

e.g., growth hormone (191 amino acids).

2) Peptides:

e.g Thyrotropin –releasing hormone (TRH) [3 amino acids]

vasopressin [9 amino acids]

oxytocin [9 amino acids]

Insulin

Glucagon

Chemical classification of hormones:

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3) Amines:

These are hormones derived from the amino acids tyrosine.

e.g.,

▪Thyroid hormones (thyroxine[T4], triiodothyronine[T3])

▪Adrenal medulla hormones (epinephrine, norepinephrine).

4) Steroids:

These are lipids derived from cholesterol.

e.g.,

▪testosterone

▪Progesterone

Chemical classification of hormones (cont):

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-There is great diversity in the biosynthesis and postsynthetic

mechanism employed in the generation of active hormones.

▪some hormones synthesized and secreted in final form;

e.g., T3

▪Others must be modified within the cell before they are secreted or before they have full biologic activity.

e.g., Insulin

[insulin synthesized in endocrine cells as inactive precursors called proinsulin (prohormones).

proinsulin, a polypeptide chain of about 80 amino acid residues, which is converted into active insulin(51 residues)

by enzymatic removal of part of the chain.

Biosynthesis of hormones:

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-Several polypeptide hormones are synthesized in endocrine cells as inactive precursors called prohormones.

-Prohormones are stored as inactive form in the endocrine cell, often in secretory granules, ready to be quickly converted into their active forms by enzymatic action when the cell receives the appropriate signal.

Hormone transport:

-The steroid and thyroid hormones bound to specific transport

(carrier) proteins.

-peptide and catecholamine hormones do not bind to carrier proteins.

Biosynthesis of hormones (cont.):

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-A target tissue defined as having a unique biochemical or

physiologic response to a hormone.

e.g.,

-the specific target tissue of TSH is thyroid gland.

-In contrast, insulin affect many tissues: insuline enhances

glucose uptake and oxidation in muscle, lipogenesis in fat,

amino acid transport in liver and protein synthesis in liver

and muscle.

A hormone may have one target tissue or it may

affect a number of tissues:

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-Some hormones yield immediate physiological or biochemical

responses e.g., adrenaline.

-Others act slowly e.g., the estrogens, thyroid hormones yield

maximal response in their target tissues only after hours or

even days. [such differences in response time correspond to a

difference in mode of action].

Physiological effects:

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Hormones act through specific high affinity cellular receptors:

-All hormones act through highly specific receptors in hormone-sensetive target cells.

-Hormone receptors are proteins.

-The hormone bind to the receptor with high affinity.

-The specificity of hormone action results from structural

complementarity between the hormone and its receptor.

-The high affinity of the interaction allows cells to respond to very low conc. of hormone.

Hormones bind to specific receptors on or in target cells.

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●Peptide and amine hormones: are water- soluble hormones (e.g.,

insulin, and epinephrine) bind to cell surface receptors. [e.g.,

epinephrine activates (through its receptor) adenylate cyclase, which

produces cAMP.

cAMP in turn activates enzymes to brakdown glycogen].

●Steroid and thyroid hormones: are water-insoluble hormones,

readily pass through the plasma membrane of their target cells to

reach their receptor proteins in the nucleus.

-with this class of hormones, the hormone-receptor complex itself

carries the message; it interacts with DNA to alter the expression of

specific gene. 14

Hydrophilic hormones bind to receptor on target cells

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-Hormones that act through plasma membrane receptors

generally triggers very rapid physiological or biochemical

responses.[e.g., just seconds after the adrenal medulla secretes

epinephrine into the blood stream, skeletal muscle responds by

accelerating the breakdown of glycogen.

-By contrast, the thyroid hormones and the steroid hormones

promote maximal responses in their target tissues only after

hours or even days.

These differences in response time correspond to

difference in mode of action. 17

●In general, the fast- acting hormones lead to a change in the

activity of one or more preexisting enzymes in the cell.

-The slower acting hormones generally alter gene expression,

resulting in the synthesis of more or less of the regulated

proteins.

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The local concentration of a hormone around the target tissue depends

upon:

1) The rate of synthesis and secretion of the hormone.

2) The binding of the hormone with specific carrier proteins in the plasma.

3) The rate of conversion of an inactive form of the hormone into the active form.

4) The rate of clearance of the hormone from blood by degradation or excretion.

-Alteration of any of these processes can result in a change of the

hormonal activity on a given target tissue.

Several factors determine the overall response of a target tissue to a

hormone:

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-Physiologic hormone levels in blood are maintained by a variety of mechanisms.

-Negative feedback control is commonly employed, especially by the hypothalamic – pituitary target gland systemes.

▪A hypothalamic releasing hormone stimulates the synthesis and release of an anterior pituitary hormone, which in turn stimulates the production of the target organ hormone.

-High levels of the target organ hormone inhibit the system by decreasing hypothalamic synthesis and action, or by decreasing pituitary hormones.

Feedback control (feedback regulation):

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What are the functions of the Endocrine System?

Name 5 Endocrine Glands and what they do.

What are hormones?

How do hormones work?

Name some hormones and what they do.

At what times in your life do you think your Endocrine System is most

active?

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