Introduction to Endocrinology Flashcards
Define endocrine system
A system that integrates + controls organ function via secretion of hormones which are carried in the blood and which they influence the activity of the target organ
Give an example where the body responds quickly to a hormone being released
Increased heart rate in response to adrenaline
Give an example where the body responds slower to a hormone being released
Increased protein synthesis in response to growth hormone
What are paracrine chemicals?
Chemicals that act locally at the site of synthesis, e.g. histamine
What are autocrine chemicals?
Chemicals that act on the same cell that synthesises the hormone, e.g. cytokines
What are exocrine chemicals?
Chemicals released from an exocrine gland via ducts to the external environment (incl. GI tract)
How do tissues detect hormones?
Via presence of receptors for that chemical in or on the cell (basically no receptor = no response)
What is involved in neural communication?
Neurotransmitters act over a synaptic cleft to influence the activity of the post-synaptic cell
What is the neuroendocrine system?
Endocrine + nervous systems combined
Nerves release hormones which enter the blood
Why do hormones only act on specific target cells despite them being distributed in the blood?
Only the specific target cells have the receptors for any specific hormone
Does 1 hormone have the exact same action on different tissues with the receptor for the hormone?
No - the same hormone can have different effects in different target cells
What is the function of hormones?
To bring about changes in the activities of their target cells/tissues
How are endocrine hormone levels controlled?
Via negative feedback loops
Some endocrine pathways also respond to neural feedback loops
What are the three classifications of endocrine hormones?
Peptide/protein
Steroid
Amine
What are peptide/protein hormones composed of?
Amino acid chains
What are steroid hormones derived from?
Cholesterol
What are amine hormones derived from?
One of two amino acids (tryptophan or tyrosine (most common))
Give examples of peptide hormones
TRH, FSH, insulin
Describe how peptide hormones are made
Synthesised in advance of need + stored in vesicles
Initial hormone produced by ribosome is v. large (preprohormone) + may have 1+ copies of the active hormone
Preprohormone cleaved in RER into smaller, (still) inactive proteins - prohormones
Prohormones packaged in vesicles in Golgi apparatus with proteolytic enzymes which break it down into active hormone + other fragments which are stored in vesicles until release is triggered and all contents are released into the plasma (co-secretion)
In which situation may measuring inactive fragments in plasma be useful?
C-peptide in diabetes to check for endogenous insulin production
What is C-peptide?
Inactive fragment cleaved from insulin prohormone
What is the ratio of production of C-peptide to insulin?
1:1
Insulin metabolised faster so levels of C-peptide 5x higher
How do peptide hormones travel in the blood?
Water soluble so easily dissolve in plasma
How do peptide hormones act on their target cells?
Bind to membrane receptors on target cell as cannot cross cell membrane
How do most peptide hormones work?
Via modulating either GPCR or tyrosine kinase linked signalling pathways
How do GPCR (G protein couple receptor) and tyrosine kinase linked signalling pathways work?
Phosphorylate existing proteins in the cell + modify their function, e.g. inactivating/activating enzymes/opening ion channels etc.
How fast do peptide hormones cause changes in the cell after they bind to their membrane receptor?
In seconds to minutes
How do GPCRs work?
Activates 2nd messenger system and/or ion channels leading to modification of existing proteins
2nd messengers may also alter gene expression
How do tyrosine kinase linked receptors work?
Alter gene expression (so slower, but longer lasting activity)
How are steroid hormones produced?
Produced as req.
Diffuse across membrane into ISF and blood
Why are steroid hormones produced as required?
They are so lipophilic that they could not be retained by a lipid membrane
How are steroid hormones transported in the blood?
Bound to carrier proteins, e.g. albumin (this protects them from enzymatic degradation and hence they have a long half life)
Name 4 organs that produce steroid hormones
Gonads - sex hormones
Placenta - hCG, sex steroids
Kidney - vitamin D3
Adrenal cortex - corticosteroids
What is the mechanism of action of steroid hormones?
Lipophilic + so cross plasma membrane, so receptors are inside cells (cytoplasmic/nuclear) and trigger the activation/repression of gene this leads to increased or decreased protein synthesis
What is the activation/repression of gene function by steroid hormones known as?
Genomic effect
Give examples of amine hormones
Dopamine, adrenaline, noradrenaline
What amine hormone is derived from tryptophan and what is its function?
Melatonin
Regulates circadian rhythm
With regards to steroid/thyroid hormone, which proportion is biologically active and can cross capillary walls to enter target cells?
The unbound, free hormone in the plasma
True or false:
The free hormone: hormone-protein complex ratio is in favour of free hormone
False
What is the Law of Mass Action?
As free hormone leaves the plasma (taken up by cells) more hormone is released from the carriers
How do you work out the total plasma hormone concentration?
Free hormone + complexed hormone
What does the hormone concentration in the blood depend on?
Rate of secretion vs rate of removal
How are hormone removed from the blood?
Excretion/metabolic transformation in liver and kidneys
Which hormones are excreted quickly?
Amines and peptides
Which hormones are excreted more slowly and why?
Steroid/thyroid hormones as they are protein bound
How can hormones influence the ability of target cells to respond?
Regulation of the number of hormone receptors:
1. upregulation (if prolonged exposure to low hormone conc.) which increases tissue sensitivity to hormone
- downregulation (after prolonged high exposure to hormone) which decreases sensitivity to hormone
What is a permissive effect?
Presence of one hormone enhances the effect of another
Give an example of a permissive effect
Epinephrine causes modest lipolysis in adipose tissue, but if thyroid hormones present greatly increased lipolysis occurs
TH –> increased synthesis of receptors for epinephrine on adipocytes
Why do you often need a 24 hour measurement for hormones?
Most hormones released in short burst so single values can be misleading
