Principles of Hormone Action

Question 1

Define Hormone

Answer 1

The term hormone (from the Greek for I arouse to activity or I was excite) was first used by Starling in 1905.
Hormones are:
One type of first (or primary) messenger
Molecules that have an effect of specific organs called target organs
Only cells with specific receptors for the hormone will respond to the hormone called target cells
Organs, tissues or cells lacking the specific receptors do not respond to the stimulating effects.

Question 2

What types of hormones are there?

Answer 2

Lipid-derived hormones
Amino Acid Derivative Hormones
Peptide Hormones

Question 3

Describe Lipid-derived hormones

Answer 3

Derived from cholesterol
Examples: sex hormones (androgens, estrogens and progesterone) and hormone produced in adrenal glands (glucocorticoids and mineral corticoids)
Most lipid hormones are derived from cholesterol and thus are structurally similar to it.
Functions: include water balance, sexual development and stress response.

Question 4

Describe amino acid derivative hormones

Answer 4

Small molecules commonly derived from tyrosine and tryptophan.
Examples: thyroid hormones (lipid soluble) and catecholamines (water soluble)
Thyroid hormones regulates the development of organs and metabolism.
Noradrenaline and adrenaline (catecholamines) increase heart rate, dilate blood vessels and cause the release of glucose during times of stress.

Question 5

Describe peptide hormones

Answer 5

Most hormones are peptide hormones
Formed from a chain of amino acids
Examples include TSH ( a few amino acids long) and a larger peptide hormone is insulin
Stored in vesicles until signalled to be released into bloodstream.
Can be very small (a few amino acids in length) or large (around 100 amino acids).
Other examples: prolactin (aids in lactation), growth hormone, vasopressin (water regulation), which are produced by the pituitary gland.
Atrial natriuretic peptide (ANP), which is produced by the heart (regulates homeostasis of the circulatory system; released in response to high blood pressure and dilation of the atrium).

Question 6

Define chemical messengers

Answer 6

Synthesized from a 20-carbon amino acid called arachidonic acid.
Degrade very easily
Do not stray far from their production site e.g. prostaglandin.

Question 7

Characteristics of chemical messengers

Answer 7

A unique characteristic in that they are produced and secreted by nearly every cell in the body instead of just one gland. Eicosanoids have various important roles in the body including inflammation, blood pressure and blood clotting,
Prostaglandins have a wide variety of functions ranging from uterine contractions to bronchodilation, inflammation and fever. Aspirin acts on prostaglandins to reduce pain and fever.

Question 8

Types of signalling

Answer 8

Endocrine
Paracrine
Autocrine

Question 9

Paracrine

Answer 9

Paracrine signalling is when neighbouring cells signals to each other.

Question 10

Autocrine

Answer 10

Autocrine signalling is when a cell sends signals to itself.

Question 11

Endocrine

Answer 11

Endocrine signalling uses circulatory system to transport ligands.

Question 12

Describe receptors

Answer 12

Receptor is a protein molecule usually found embedded within the plasma membrane surface of a cell that receives chemical signals from outside the cell.
Binding of a ligand to a receptor changes it shape or activity, allowing it to transmit signal or directly produce a change inside of the cell.

Question 13

Two categories of receptors

Answer 13

Receptors come in many types, but can be divided into two categories:
Intracellular receptors: inside of the cell (in the cytoplasm or nucleus)
Cell surface receptors: found in the plasma membrane

Question 14

Describe intracellular receptors

Answer 14

Hydrophobic signalling molecules (ligands) diffuse across the plasma membrane
Interact with intracellular receptors in the cytoplasm (or nucleus).
Example: the primary receptors for hydrophobic steroid hormones such as the sex hormones estradiol (an estrogen) and testosterone are intracellular.
Receptors function as ligand-dependent transcription factors.

Question 15

How do hormone interact with intracellular receptors?

Answer 15

The hormone-receptor complex binds to promoter regions of responsive genes and stimulate/inhibit transcription of those genes.
The mechanism of action of steroid hormones is to modulate gene expression in target cells. By selectively affecting transcription from a battery of genes, the concentration of those respective proteins are altered, which change the phenotype of the cell. The response is not immediate!

Question 16

Describe cell-surface receptors

Answer 16

Performs signal transduction, converting extracellular signal into an intracellular signal
Three main components:
1. An external ligand binding domain - extracellular domain
2. A hydrophobic membrane spanning region
3. Intracellular domain inside the cell

Question 17

Three categories of cell surface receptors

Answer 17

There are three general categories of cell surface receptors:
G protein-linked receptors
Enzyme-linked receptors
Ion-channel linked receptors

Question 18

G-protein Linked Receptors

Answer 18

Ligand binds to receptor, this activates the membrane protein called G protein.
The activated G protein then interacts with either an iron channel or an enzyme in the membrane
Each receptor has its own specific extracellular domain and G-protein binding site.

Question 19

Enzyme-Linked Receptors

Answer 19

Cell-surface receptors with intracellular domains that are associated with an enzyme
Normally have large extracellular and intracellular domains.
When a ligand binds to the extracellular domain, a signal is transferred that activates the enzyme component of the receptor which leads to a response.

Question 20

Ion-channel Linked Receptors

Answer 20

Ligand binds to a receptor
Conformational change in the protein structure that allows ions such as sodium, calcium, magnesium and hydrogen to pass through
Change the activity of ion-binding enzymes and voltage-sensitive channels, to produce a response.

Question 21

Types of second messengers

Answer 21

Second Messenger:
Cyclic AMP
Protein Kinase activity
Calcium and/or phosphoinositide
Cyclic GMP

Question 22

Downregulation of receptors

Answer 22

Downregulation of receptors: receptors chronically exposed to an excessive amount of a ligand (endogenous mediators or from exogenous drugs).
This results in ligand-induced desensitization or internalization of that receptor.

Question 23

Upregulation of receptors

Answer 23

Upregulation of receptors: super-sensitized cells especially after repeated exposure to an antagonistic drug or prolonged absence.

Question 24

Receptor agonist

Answer 24

Receptor agonist: down regulation of their respective receptors

Question 25

Receptor antagonist

Answer 25

Receptor antagonist: upregulation of their respective receptors

Question 26

Pulsatility

Answer 26

Hormone secretion often follows a rhythm or is pulsatile
It is secreted in a burst-like or episodic manner rather than constantly

Question 27

Direct effects of hormones

Answer 27

Direct effects are the result of a growth hormone (GH) binding receptor on target cells. Fat cells have GH receptor and GH stimulates them to break down triglyceride and supresses their ability to take up and accumulate circulating lipids.

Question 28

Indirect effects of hormones

Answer 28

Indirect effects are mediated primarily by insulin-like growth factor-I (IGF-I), a hormone that is secreted from the liver and other tissues in response to GH. The majority of the growth promoting effects of GH are actually due to IGF-I acting on its target cells.