Introduction to endocrine Flashcards
Define what the endocrine system is
A system that integrates and controls organ function via the secretion of chemicals (hormones) from cells, tissues or glands…
…which are then carried in the blood to target organs, distal from the site of hormone synthesis, where they influence the activity of that target organ
In a basic sense, what are the differences between the following:
a) endocrine
b) paracrine
c) autocrine
d) exocrine
Endocrine - hormones released into blood
Paracrine - act locally to the site of synthesis/secretion (eg histamine)
Autocrine - acts on the same cell that produced/secreted it (eg cytokines)
Exocrine - secreted to outside environment via ducts
In order for endocrine communication to happen, there must be _________ on the cells of the target tissue
Receptors
Tissues detect hormones through the presence of specific receptors for that chemical on/in the cells. No receptor = no response
How does neuroendocrine communication work?
Nerves release hormones which enter blood and travel to their target cells e.g. hypothalamic – posterior pituitary axis.
What are the different types of endocrine hormones?
What are they derived from/made from?
Peptide or protein hormones (most common) - amino acid chains
Amine hormones - derived from tyrosine or tryptophan
Steroid hormones - all derived from cholesterol
How are peptide hormones produced?
Synthesised in cells as preprohormone in advance of need then cleaved into prohormone and stored in vesicles until required
Describe what happens in stage (1)
Preprohormone polypeptide synthesised from mRNA strand
The chain contains a ‘signal sequence’ of amino acids - which causes it to be directed into the ER’s lumen
What happens at stage (2)
Enzymes in the Endoplasmic reticulum chop off the signal sequences from the preprohormone to make a prohormone (still inactive)
The prohormone is then packaged into a transport vesicle and moves towards to golgi
What happens at stage (3)
the prohormone moves through the golgi complex and passed out the other end in a secretory vesicle
What happens in stage (4)?
Secretory vesicles bud off the golgi complex
These contain the prohormone and enzymes - which chop the prohormone into 1 or more active peptides (+ fragments)
What happens in stage 5 & 6?
Secretory vesicle releases its contents into ECF through exocytosis. This is called co-secretion - as both hormone and fragments are yeeted out.
The hormone then moves from the ECF to the blood to be transported to its target
Co-secretion involves the secretion of hormones alongside their inactive fragments.
Give an example of how measuring the levels of these inactive fragments be clinically useful
C-peptide levels can be measured for diabetic patients
(measuring insulin levels does not differentiate between endogenous and synthetic insulin)
How do peptide hormones travel around the body?
Peptide hormones are water-soluble (hydrophilic) so dissolve in plasma but not across membranes.
This means receptors for peptide hormones are on target cell membranes, not intracellular
How long does it take peptide hormones to produce a biological response?
Generally fast-acting - (seconds to minutes)
What signalling pathways are peptide hormones typically involved with?
Most peptide hormones work via modulating either 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 GCPR signalling pathway?
G-coupled protein receptors - found on membrane
Hormone binding activates 2nd messenger system and/or ion channels leading to modification of existing proteins.
Rapid response
What is the effect of hormones binding to Tyrosine kinase linked signalling pathways?
Alters gene expression - (although GCPR 2nd messengers can do this)
Slower, longer lasting activity
What are amine hormones?
Hormones derived from amino acid Tyrosine - apart from melatonin which is derived from tryptophan
Examples - Dopamine, Norepinephrine (NA), Epinephrine (adrenaline), Thyroxine, Triiodothyronine
The amine hormones derived from tyrosine can be split into two groups.
What are these groups?
Catecholamines & Thyroid hormones
Catecholamines - Dopamine, Adrenaline, NA:
- Hydrophilic
- Similar mechanisms of action to peptide hormones
Thyroid hormones - Thyroxine, Triiodothyronine:
- Lipophilic
- Similar mechanisms of action to Steroid hormones.
What are steroid hormones derived from?
Hormones derived from cholesterol
How are steroid hormones stored?
Steroid hormones arent really stored - they are synthesised directly when needed
This is because they are lipophilic so just diffuse across membranes (cant be contained) into the ISF, blood
How are steroid hormones transported around the body?
Being poorly soluble in water - they are transported bound to carrier proteins such as albumin.
This stabilises their transport through the plasma and protects them from enzymatic degradation, phenomenally increasing their half life (60-90mins vs 2mins for amine hormones).
What organs/tissues produce steroid hormones?
Gonads (testes/ovaries) - Sex steroids
Placenta - Human Chorionic gonadotropin (hCG), sex steroids
Kidney - vitamin D3
Adrenal cortex - corticosteroids (cortisol, aldosterone)
What determines what steroid hormone is produced from cholesterol?
Depends on what cell (what tissue) is doing the altering - gonad cells will do different stuff than kidney etc
Where are receptors for steroid hormones located?
Intracellularly - either cytoplasmic or nuclear
Activation will cause activation or repression of gene function within that cell (genomic effect) - leading to increased/decreased synthesis of specific proteins
This takes a while compared to the other hormone types
Hello, this diagram is of how steroid hormones work. Describe what happens at each stage
When talking about lipophilic hormones such as steroid hormones - we need to know what ‘free hormones’ are
What does this mean?
Lipophilic hormones are mainly transported in plasma bound to transport proteins - such as albumin
However - a small portion of the steroid hormones remain unbound (free)
The ratio of free to complexed steroid hormone is usually very small. The extent of protein binding can have important effects on the hormone’s actions
In your plasma, most steroid hormones are complexed with albumin and a small amount is not
What happens when the free steroid hormones are taken up by cells?
Law of mass action states that when free hormone leaves the plasma - that more hormone is released by the carriers
Thus the concentration of free hormone remains the same (ish)
Give examples of the carrier proteins used to transport lipophilic hormones such as steroids or thyroid hormones
Some specific transporters:
- corticosteroid-binding globulin
Non-specific (carry lots of different ones):
- Albumin
What are the benefits of using transporter proteins to transport lipophilic hormones?
Increases solubility –> required for blood-mediated transport
Protects from degradation –> increased half life (> 60 min)
In most endocrine pathways, secretion is responsive to _______ _______ reflexes
In most endocrine pathways, secretion is responsive to negative feedback reflexes
An example of this is parathyroid hormone - its release stimulates increased plasma [Ca2+], but increased [Ca2+] inhibits the release of parathyroid hormone
Why are single measurements of hormone levels in a patients blood not very useful in many ways?
Hormones tend to be released in short bursts - thus [hormone] varies widely depending on the environment, time etc
Clinical relevance is that single values of [hormone] may be misleading
