Introduction to the Endocrine System Flashcards
Is the endocrine system anatomically continuous?
No
Give an example of fast and slow endocrine responses.
Fast - Increase in HR due to adrenaline
Slow - Increase in protein synthesis die to Growth Hormone
Give some sites of principle endocrine glands.
Hypothalamus Pituitary Thyroid Parathyroid Adrenal glands
Renal medulla, renal cortex and in the kidney’s themselves
GI tract
Ovary and testes
What are the features of an endocrine hormone?
Produced by a cell/group of cells where is secretes into the blood where it is transported to different targets by binding to target receptors
Do endocrine hormones exert effects at high or low concentrations?
Low - range if 10^-9M to 10^-12M
What often terminates the endocrine hormones actions?
Neg. feedback loops
What are some other chemicals that endocrine hormones should NOT be confused with?
Paracrine
Autocrine
Exocrine
What do paracrine hormones/chemicals do? Give an example of one.
They act local to the site of synthesis rather than travel to distal sites like endocrine does
Histamine
What do autocrine hormones do?
Example?
They act in/on the same cell that synthesised it
Cytokines
What do exocrine chems do?
Example?
Released from exocrine glands via ducts to the external environment (including the GI tract as it is classed as external)
Saliva
Sweat
Bile
What does the endocrine system do in conjunction with the nervous system?
Communicates with and controls all body functions
What is the difference between an endocrine hormone and a neurotransmitter?
A neurotransmitter acts on the post-synaptic cell to influence its activity but this act is local and not distal
What is a neuroendocrine system?
NS and endocrine combines - nerves rleases hormones which enter blood and travel to target cells
Example of a neuroendocrine system?
Hypothalamus -posterior pituitary axis
How specific is the response of the target cell to hormones?
Very - if the target cell doesn’t have the right receptor, hormones won’t act on it
Can hormones act on different cells with different effects?
Yes
Example of a hormone acting on different cells with different effects?
Insulin acts on skeletal muscle/adipose tissue to increase glucose uptake
But acts on liver to increase glycogenesis (make glycogen) and decrease gluconeogenesis
What is gluconeogenesis?
Synthesising glucose from non-carb substrates like amino-acids
What is glycogenolysis?
Breaking down glycogen to form glucose
What are the 3 types of hormone?
Peptide
Steroid
Amine
Give examples of a peptide hormone?
Insulin
Thyrotropin-releasing hormone
Follicle stimulating hormone
What makes up a peptide hormone?
Amino acids
Which type of hormone is most common?
Peptide
Describe the sysnthesis of a peptide hormone.
An inactive preprohormone is made by ribosomes
Preprohormones have 1 or more copies of the active hormone in the structure, it is then cleaves by the rough ER into a prohormone(s) and is still inactive
Prohormones are then packed into vesicles in the golgi apparatus along with proteolytic enzymes that break the prohormone down into an active hormone and other fragments
Once the preprohormone becomes the hormone, what happens?
The hormone and fragments are stored in vesicles til needed - when needed entire vesicle contents emptied into blood
What clinical relevance do the fragments have?
Measuring the inactive fragments in plasma is useful to get the exact amount of hormone the body is making
Example - for diabetics who take insulin, measuring insulin in blood is not an accurate way of determining how much insulin the body make, but measuring the fragments - C-peptide - is an accurate way
Is there a 1:1 ratio between c-peptide and synthesised insulin?
No since insulin is metabolised faster it is actually 5x higher than c-pep
When the preprohormone become a prohormone - what is the part that is cleaved off called and what is it’s job?
Called a signal sequence - it directs the chain to the endoplasmic reticulum
Are peptide hormones hydrophobic/lipophillic or hydrophillic/lipophobic?
Hydrophilic/lipophobic
How do they travel in blood?
Since hydrophilic they do not need carriers and just dissolve in blood
If peptide hormones are lipophobic, what does this mean for its mechanism of action?
They can’t cross cell membranes, so must bind to target receptors on the surface of the membrane
Do peptide hormones have a generally fast or slow response once bound?
Fast
What are the 2 pathways used to cause a change in the cell once hormone is bound? What do these pathways cause?
G-protein coupled receptors
Tyrosine kinase linked signalling pathways
Both pathways phosphorylate proteins in cell causing a change
Explain the G-protein CR pathway
Once hormone binds - activates 2nd messenger system (a cascade) and/or opening/closing of ion channels
This leads to modification of proteins and a rapid response
Explain the Tyrosine kinase linked receptor
Alters gene expression of proteins either activating or de-activating the gene
Is a slower response time
Can the GPCR pathway also alter gene expression? If so, how?
Yes, the 2nd messengers can alter gene expression
What is an issue with peptide hormones?
They have a short half life and are vulnerable to degradation - must be continuous secreted to have an effect
What are steroid hormones derived from?
Cholesterol
How are different steroid hormones made if they are all derived from cholesterol?
The type of steroid hormone made depends on the fact that different cells have different enzymes synthesizing differing derivatives of cholesterol
Are steroid hormones stored?
No only made as needed
Why are steroid hormones not stored?
They are hydrophobic/lipophillic so can’t be kept within the cell membrane
How do they travel in blood if they are hydrophobic?
Carriers such as albumin
What can albumin do for the hormone?
Increase half-life of it
What hormones does the kidney make?
Kidneys make Vit. D3
Mechanism of action for steroid hormones?
Since they are lipophilic, receptors are inside cell
They alter gene expression - activation or repression which alters protein sysnthesis
Steroid hormones have a slow or fast response?
Slow when they bind inside cell
Some evidence suggests they can bind on membrane for a rapid effect
What are amine hormones derived from?
2 amino acids
What are the 2 amino acids that amine derives from?
Tyrosine
Tryptophan
Which of the 2 amino acids makes all but 1 amine hormone?
Tyrosine
What amine hormone does tryptophan make?
Melatonin
What does melatonin do?
Regulates circadian rhythm
What are the 2 subcatagories of amine hormone that tyrosine makes?
Catacholamines
Thyroid hormones
Name some catacholamines and where they are made
Dopamine - brain
Noraadrenaline - neurons
Adrenaline - adrenal gland
Mechanism of action for catacholamines?
They are hydrophilic/lipophobic so have same mechanism as peptide hormones
Are catacholamines vulnerable to degradation?
Yes - also need to be continually secreted
Name some thyroid hormones.
Thyroxine
Triiodothyronine
Mechanism of action for thyroid hormones?
Hydrophobic/lipophilic so same as steroid
Why is it important that there is a small amount of unbound free steroid/thyroid hormone in plasma at any given time?
Only unbound can cross membrane
So a ratio exists between bound and unbound and is constantly maintained.
Example - free hormone degrades due to short half life or binds to receptor = bound hormone to unbind = more homrone produced to replace bound hormone lost
What is the law of mass action?
As free hormone leaves the plasma more hormone is released from carriers
Bound:unbound ratio favours what side?
Bound is higher
In health does the level of free hormones remain constant or vary?
Constant
How much hormone is needed for a repsone?
Minute amounts
How is total plasma hormone conc calculated?
Free hormone + complex bound hormone
What type of hormone carriers are there?
Specific - corticosteroid binding globulin (only binds to one hormone)
Non-specific - albumin
How are hormones removed from body?
Excreted or metabolically transformed via liver and kidneys
What hormones are excreted easily?
Peptide and catacholamines since short half life
What hormones take longer to excrete?
Steroids and thyroid hormones since they are protein bound
What are 2 ways hormone secretion is controlled?
Neg feedback loops - high levels of hormone feeds back and stops production
Neural feedback loops - sympathetic response causes adrenaline secretion
What happens when there is a prolonged exposure of low hormone conc. on a cell?
The cell will make more receptors to increases sensitivity to the hormone
What happens when there is a prolonged exposure of high hormone conc. on a cell?
The cell decrease its receptors which in turn decreses sensitivity to the hormone
This change in tissue sensitivity can affect receptors for other hormones too - T or F
True - Can have permissive or antagonistic effects
Explain a permissive effect
The presence of one hormone enhances the effect of another
For example adrenaline causes a lower rate of lipolysis in adipose tissue than when adrenaline and thyroid hormone are combined = greater lipolysis
This is because the presence of thyroid hormone causes cell to make more adrenaline receptors - thyroid hormone has no effect on lipolysis but is permissive to adrenaline
Hormones are released in short bursts and conc. varies widely. Why is this clinically relevant?
A single reading may be misleading - need to monitor over 24 hrs
Look at full hormone table on slide
Ok
