Endocrine intro Flashcards
Define the endocrine system
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
Define a neurohormone
any hormone produced and released by neuroendocrine cells into the blood
Define a neurohormone
any hormone produced and released by neuroendocrine cells into the blood
once in the blood, acts like any other endocrine hormone
How are autocrine hormones different from endocrine hormones + example of an autocrine hormone
act on/in the same cell that synthesises the hormone e.g. cytokines
unlike endocrine hormones which are secreted into the blood and act distally
How are paracrine hormones different from endocrine hormones
They act locally by diffusing from its source into the ECF to neighbouring target cells , unlike endocrine hormones which are secreted into the blood and act distally
Endocrine system works together with what system to control all body functions
Nervous
Compare and contrast the main features of nervous and endocrine modes of communication
Hormones travel in the blood to their target organs/tissues
Neurotransmitters released from presynaptic neurons travel across the synaptic cleft to the postsynaptic cell to influence its activity (acts locally)
All cells are exposed to hormones however only those that have what produce a response to the hormone
Specific receptor for that hormone
only target cells have this
Classify hormones according to their chemistry (3)
Peptide/protein hormones (most common type)
Amine hormones
Steroid hormones
Basic function of hormones
bring about changes in the activity of their target cells and tissues, (increase/decrease a particular activity
What glands are part of the endocrine system (7)
pineal gland thyroid gland, parathyroid glands adrenal glands, pituitary gland, hypothalamus, pancreas
6 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 by binding to receptors on target tissues
Have their action terminated, often via negative feedback loops
Why does it not matter that endocrine hormones are only secreted in very low physiologically active concentrations
Target cells are highly sensitive to hormones so only a small conc. needed to bring about a response
What are peptide hormones composed of
chains of amino acids
What 2 amino acids are amine hormones derived from
Tryptophan (only 1 amine hormone derived from this) or tyrosine (majority derived from this)
What are steroid hormones derived from
Cholesterol
How are peptide hormones synthesised
Synthesised as preprohormone (inactive) by ribosomes in advance of need then cleaved into prohormone (still inactive) and stored in vesicles until required
Also stored with proteolytic enzymes in the vesicle which breaks the pro hormone down into active hormone + other fragments
Define a preprohormone
initial peptide hormone produced by ribosomes that’s large and inactive
What do enzymes in the endoplasm reticulum do to a preprohormone
cleave it into smaller units to leave smaller but still inactive proteins called prohormones
What inactive fragment can clinically useful to measure in the plasma in diabetes
C-peptide
What is C-peptide + what does it indicate
inactive fragment cleaved from the insulin prohormone
Levels of C-peptide in plasma or urine are often measured to indicate endogenous insulin production from the pancreas
Which is metabolised faster - insulin or C-peptide (inactive fragment cleaved from insulin prohormone)
Insulin
Mechanism of action of peptide hormones
- are they hydrophilic/hydrophobic + what implications does this have
- what kind of receptor do they activate
They’re water soluble so dissolve in plasma making transport via blood easy
However this means they can’t cross cell membrane so bind to membrane bound receptors on target cell
Activate GPCR (G protein coupled receptor) or tyrosine kinase linked signalling pathways –> these pathways phosphorylate existing proteins in the cell and modify their function
Describe how G protein coupled receptors work once activated + is the response slow or rapid
Activates 2nd messenger system and/or ion channels leading to modification of existing proteins
Rapid response
Describe how tyrosine kinase linked receptors work once activated + is the response slow or rapid
Alters gene expression
Slower, longer lasting activity
What is the only amine hormone derived from tryptophan
Melatonin
Amine hormones derived from tyrosine can be divided into 2 subtypes - name them + name examples of each subtype
Catecholamines, e.g. dopamine, norepinephrine
Thyroid hormones, e.g. thyroxine
Catecholamines (a subtype of amine hormone derived from tyrosine) have a similar mechanism of action to what other class of hormones
Peptide hormones
- as catecholamines are also hydrophilic
Thyroid hormones (a subtype of amine hormone derived from tyrosine) have a similar mechanism of action to what other class of hormones
Steroid hormones
-as thyroid hormones are also lipophilic
Unlike other types of hormones, steroid hormones are … rather than being stored and released when needed
synthesised directly as needed
Why are steroid hormones synthesised directly as needed rather than being stored and released when needed
because they are highly lipophilic (lipid soluble) so cannot be retained within lipid membranes
Are steroid hormones hydrophobic or hydrophilic
hydrophobic
Since steroid hormones are hydrophobic, how are they transported in the blood
bound to carrier proteins such as albumin. This stabilises their transport through the plasma and protects them from enzymatic degradation
Half life of steroid hormones vs half life of amine hormones
60-90mins
2 mins - amine hormones
Steroid hormones are produced by (4)
Gonads (testes and ovary)
Placenta
Kidney
Adrenal cortex
What steroid hormones do the following produce
- gonads
- placenta
- kidney
- adrenal cortex
Sex steroids
hCG, sex steroids
vitamin D3
corticosteroids
Since steroid hormones are hydrophobic, how are they transported in the blood
bound to carrier proteins such as albumin.
This increases their solubility and stabilises their transport through the plasma and protects them from enzymatic degradation
The steroid hormone ultimately produced depends on what
on where is it produced and what enzyme that has synthesised it from cholesterol
Mechanism of action of steroid hormones
- are they lipophobic/lipophilic
- where is their receptor located
- what happens once receptor stimulated
Lipophilic so they can cross membrane easily
As such their receptors are located inside cells (cytoplasmic or nuclear receptors) and trigger either activation or repression (inhibition) of gene function within the nucleus = genomic effect
So protein synthesis is either increased or decreased
Do steroid hormones produce a rapid or slow response
slow process so there is a lag time between hormone release and biological effect (hours to days) but effect persists for around the same time
Most steroid and thyroid (a subtype of amine hormone derived from tyrosine) hormones (i.e. lipophilic hormones) in the plasma are bound to proteins, however…
There is a small amount of unbound free steroid/thyroid hormone in the plasma which is the physiologically important fraction
Steroid and thyroid hormones bound to protein are not physiologically active and can’t…
diffuse across capillary walls to target cells; only free hormone can
Total plasma [hormone] =
[free hormone] + [complexed hormone]
What hormones are hydrophobic (so poor solubility in plasma)
Steroid Thyroid (subtype of amine hormones derived from tyrosine)
Peptide and catecholamine hormones are water soluble so vulnerable to what
degradation before they reach their target
short half life in plasma
What hormones are water soluble (lipophobic/hydrophilic)
Peptide
Catecholamine
For the majority, hormone secretion is controlled/stopped by…
negative feedback reflexes
As well as endocrine pathways responding to negative feedback reflexes to tell it to stop producing more hormone, they also respond to…
neural feedback loops, e.g. adrenaline
Prolonged exposure to low [hormone] in the plasma often leads to… + purpose of this
up-regulation of receptor number (which increases tissue sensitivity to hormone).
to maintain homeostasis
prolonged exposure to high [hormone] in the plasma typically leads to… + purpose of this
down-regulation of receptor number (which decreases tissue sensitivity to hormone).
to maintain homeostasis
What is a permissive effect in terms of hormones
when the presence of one hormone enhances the effect of another
What is an antagonistic effect in terms of hormones
when the presence of one hormone reduces the effect of another
Most hormones are released in short bursts so [hormone] varies widely therefore single values of [hormone] may be misleading, so what is the best way of measuring this
24 hour monitoring
