Introduction to the Endocrine System Flashcards
What is the endocrine sytem?
System that integrates and controls organ function via the secretion of chemicals from cells, tissues or glands where they can be carried in the blood to target organs distal from the site of hormone synthesis, where they influence the activity of that organ
What should endocrine hormones not be confused with?
Paracrine chemicals
Autocrine chemicals
Exocrine chemicals
What are paracrine chemicals?
Chemicals that act local to the site of synthesis, do not travel to distant sites e.g. histamine
What are autocrine chemicals?
Chemicals that act on/in the same cells that it is synthesised
What are exocrine chemicals?
Chemicals that are released from exocrine glands via ducts to the external environment
Which system does the endocrine system communicate with?
Nervous system
Where do hormones travel?
In the blood
What site do hormones reach?
Site distal to that where the chemical is made
How do tissues detects specific hormones?
Through the presence of specific receptors for that hormones
What if there is no receptor for the hormone on that cell?
There will be no response
Where are neurotransmitters release from?
Pre-synaptic neurons
Where do neurotransmitters travel?
Across the synaptic cleft to the post synaptic neuron
What is the difference between hormones and neurotransmitters?
They act locally in the synaptic cleft whereas hormones travel to distal sites
When the endocrine and nervous system combine what do nerves release?
Hormones which enter the blood and travel to their target tissue
Why is the response to one hormone highly specific even though hormones travel throughout the body in blood?
Because only target cells have specific receptors for that hormone
Is the effect of one hormone the same on all cells with the receptor?
The same hormone can have different effects in different target cells
How do hormones generally change activity in a cell?
Switch enzymes on or off
What are the sits of primary endocrine glands?
Hypothalamus Pituitary Thyroid Parathyroid Adrenal glands Kidneys Medulla Cortex
What are the features of an endocrine hormone?
- Produced by a cell or group of cells
- Secreted from those cells into the blood
- Transported via the blood to distant targets
- Exert their effects at very low concentrations (act in the range 10-9 -10-12 M)
- Act by binding to receptors on target tissues
- Have their action terminated, often via negative feedback loops.
What are the classifications of endocrine hormones?
Peptide or protein
Steroid hormones
Amine hormones
What are peptide hormones composed from?
Amino acid chains
What are steroid hormones derived from?
Cholesterol
What are amine hormones derived from?
Two amino acids
Where are peptide hormones synthesised?
In advance and stored in vesicles
What are preprohormones?
Initial protein produced by ribosomes
Contain one or more copies of the active hormones in their amino acid sequence
Where are preprohormones cleaved?
On the RER
What are preprohormones cleaved into?
Prohormones
Where are prohormones packaged into vesicles?
In the golgi apparatus
What are prohormones packaged with?
Proteolytic enzymes
What do the proteolytic enzymes do to the prohormones?
Break down the prohormone into active hormone and other fragments
Where are vesicle contents of peptide hormones released?
Into the plasma
What is C-peptide?
Inactive fragment that is cleaved from insulin prohormone
Why are C-peptide levels measured?
Levels of C-peptide in plasma or urine are often measured to indicate endogenous insulin production from the pancreas (produced in equal amounts).
Why can peptide hormones not easily cross the cell membrane?
Because they are lipophobic
What is a benefit to peptide hormones being hydrophillic?
Then can transport in the blood easily
How do most peptide hormones work?
Via modulating either the GPCR or tyrosine kinase linked signalling pathways. These pathways phosphorylate existing proteins in the cell and modify their function e.g. open or close ion channels, activate or inactivate enzymes.
What is the drawback to peptide hormones being hydrophilic?
They cannot diffuse across the cell membrane
Why do peptide hormones bind to a cell surface receptor?
Because they are lipophobic and cannot readily cross the cell membrane
What 2 types of receptors will peptide hormones typically bind to?
Tyrosine kinase linked
G protein coupled receptors
What do G protein coupled receptors typically do?
Activate a 2nd messenger system
What do tyrosine kinase linked receptors typically do?
Alter gene expression
Where are steroid hormones derived from ?
Cholesterol
When are steroid hormones synthesised?
As they are needed
Are steroid hormones typically stored?
No
Why are steroid hormones not pre made then stored?
Because they are highly lipophilic and would just diffuse out of the cell membrane
Once synthesised where do steroid h. travel?
Diffuse out of the membrane into the ISF then into the blood
Why are steroid h. bound to plasma proteins?
Because they are hydrophobic
What is the effect of binding steroid h. to plasma proteins?
Stabilises their transport through the blood and protects them from enzymatic degradation
Why is there a longer 1/2 life of steroid hormones?
Because they are bound to proteins protecting them
Which organs produce steroid hormones?
Gonads
Kidneys
Adrenal cortex
Placenta
What determines which steroid is produced in which cell?
Different types of enzymes synthesising different types of cholesterol
Where are steroid hormone receptors located?
Inside cells (cytoplasmic or nuclear receptors)
What effect do steroid hormones have on the cell?
Either trigger or repress gene function within the nucleus
Consequently the either upregulate or downregulate protein synthesis
What do genes ultimately control?
The synthesis of protein
Do steroid hormones exert a fast response?
No long
Which is the only amine hormone not derives from tyrosine?
Melatonin
From which amino acid are most amine hormones derived from?
Tyrosine
Which amino acid is melatonin derived from?
Tryptophan
What does melatonin regulate?
Circadian rhythm
Where is dopamine released?
From the brain
Where is noriepinepherine released?
From neurons
Where is epinephrine released from?
Adrenal medulla
Name 3 amine hormones?
Dopamine
Noriepinepherine
Epinephrine
Which form of steroid hormone is active?
The unbound fraction
What does the law of mass dictate abut steroid hormones?
That as free hormone is taken up by the cell
More hormone is released from the carriers
What does free hormone + complexed hormone =
Total plasma (hormone)
Is a lot of steroid hormone unbound?
No only a small ratio
Are peptide hormones easily degraded?
Yes they are very vulnerable to degradation
How is prolonged action achieved with peptide hormones?
Continued secretion
What does the concentration of hormone in the blood depend on?
The rate of secretion and the rate of removal
Where does hormones removal usually occur?
In the liver or kidneys
How are most endocrine pathways controlled?
Through negative feedback
When levels are high what happens in negative feedback?
Negative feedback switches off production
What do some endocrine pathways respond to?
Neural feedback
How can hormones influence the ability of the target cell to respond?
Regulate the number of hormone receptors
What is often seen after prolonged exposure to low concentration hormone?
Upregulation in the number of hormone receptors on the target cell
What is often seen after prolonged exposure to high concentration hormone?
Down regulation in the number of hormone receptors on the target cell
What is meant by permissive effects?
When the presence of one hormone enhances the effect of another
Give an example of permissive effects
Epinephrine in the presence of thyroid hormones creates greatly increased lipolysis
