Introduction to the Endocrine System Flashcards
What is the function of the endocrine system?
Integrates and controls organ function by the secretion of hormones
What are hormones?
Chemicals released from cells, tissues or glands
How are hormones transported around the body?
Blood
What is meant by endocrine commuinication?
Hormones travelling through the blood to their target organs/tissues
Describe the difference between neurotransmitters and endocrine hormones.
Neurotransmitters- released by nerves, cross the synapse and have local site of action.
Endocrine hormones- released by cell, travel in blood and have a distal site of action
What happens if a chemical is neuroendocrine?
Nerves release hormones which enter the blood and travel to target cells
Give an example of a neuroendocrine tissue.
Hypothalamus
The response of target cells to a hormone is very specific. However, the same hormone can have different effects in different cells.
For example, what effect does insulin have when it binds in skeletal muscle/adipose tissue?
Increases glucose uptake
The response of target cells to a hormone is very specific. However, the same hormone can have different effects in different cells.
For example, what effect does insulin have when it binds in the liver?
Increased glycogenesis
Decreased gluconeogenesis
Endocrine glands are ductless.
Exocrine glands are ducted.
How do exocrine glands work?
Deliever their secretions to the external environment including the GIT
Give some examples of exocrine secretions.
Sweat, bile, saliva
Which type of glands have ducts?
Exocrine
Which type gland are ductless?
Endocrine glands
What is meant by autocrine secretions?
Cells secrete chemicals that bind to receptors on the same cell
Give an example of a type of cell which produces autocrine secretions.
Cytokines
What is meant by paracrine secretions?
Chemicals diffuse into the ECF to affect neighbouring cells
Give an example of a cell which uses paracrine secretions.
Histamine
Which type of hormonal secretions do not travel in the blood?
Autocrine and paracrine secretions.
List some of the glands which have a purely endocrine function.
Thyroid gland
Adrenal gland
Pituitary gland
Hypothalamus
Do hormones work at high or low concentrayions?
Low concentrations
->tiny changes in hormone levels can change function dramatically
Describe the patter of endocrine hormone release.
Often released in short bursts
->this means 24hr monitoring is clinically required to get a realistic picture
Name the three classes of hormone/
Peptide/protein hormones
Amine hormones
Steroid hormones
Which classification of endocrine hormone is the most common?
Peptide/protein hormones
What are peptide/protein hormones composed of?
Chains of amino acids
What are amine hormones composed of?
All derived from either tryptophan or tyrosine
In reality, only one hormone is derived from tryptophan and the rest come from tyrosine.
What is the one hormone derived from tryptophan?
Melatonin
What are steroid hormones dervived from?
Cholesterol
Peptide hormones are initially synthesised as what in advance of need?
Preprohormones
What happens to the preprohomones?
Cleaved into prohormones and stored in vesicles until required
Where are the preprohormones synthesised?
Ribosomes
Where are prohormones packaged into vesicles?
Golgi apparatus
What else is packaged into the vesicles alongside the prohormone?
Proteolytic enzymes
What role do the proteolytic enzymes play?
Breaks prohormone into active hormone and other inactive fragments
->worth noting that sometimes clinically it is more useful to measure the inactive fragments rather than the active ones, often done in measuring C-peptide in diabetes
What is C-peptide?
Inactive fragment cleaved from insulin prohormone
Why is C-peptide measured?
Indicated endogenous insulin production from pancreas
->can be measured in plasma or urine
How do the levels of C-peptide compare to levels of endogenous insulin and why?
Levels of C-peptide usually 5x higher.
This is because insulin is metabolised faster.
Why would you measure C-peptide levels and not insulin levels when determining pancreas function?
In those w Type 1 diabetes, they will be taking synthetic insulin which will also be measured and does not relate to pancreas function
Are peptide hormones water soluble?
Yes
->this means they dissolve easily in plasma making these hormones easily transportable in the blood
As peptide hormones are water soluble, this means they cannot simply cross cell membranes.
How do they do it?
Bind to receptors on the target cell
Which type of receptors do most peptide hormones target?
G-protein coupled receptor
Tyrosine kinase linked receptors
GCPR and tyrosine kinase linked receptors result in what?
Phosphorylation of intracellular target proteins, changing their activity.
Do peptide hormones generate fast or slow responses?
Fast
Are water-soluble peptide hormones hydrophobic or hydrophilic?
Hydrophilic
RECAP- which enzymes phosphorylate target proteins?
Kinase enzymes
In terms of peptide hormone signal transduction, hormones coupled to which type of receptor leads to rapid responses?
G-coupled protein receptors (GCPR)
In terms of peptide hormone signal transduction, hormones coupled to which type of receptor leads to slower, long lasting responses?
Tyrosine kinase linked receptor
RECAP- the majority of amine hormones are dervobed from which amino acid?
Tyrosine
List some amine hormones.
Dopamine, norepinephrine, epinephrine (catecholamines)
Tetraiodothyronine, triiodothyronine (thyroid hormones)
Which other type of hormones do catecholamines have similar mechanisms to?
Peptide hormones
Which other type of hormones do thyroid hormones have similar mechanisms to?
Steroid hormones
Which type of hormone is synthesised when required and not stored?
Steroid hormones
Why are steroid hormones only synthesised when required?
They are highly lipophilic so cannot be retained within lipid membranes
Steroid hormones are poorly soluble in water. Therefore, how are they transported?
Bound to proteins such as albumin
Describe the onset of action of steroid hormones
Slow onset but action lasts much longer than peptide or amine hormones
What produces steroid hormones?
Gonads (testes and ovaries)
Placenta
Kidney
Adrenal cortex
Which type of steroid hormones are produced in the gonads?
Sex steroids
Which type of steroid hormone are produced by the placenta?
hCG
Sex steroids
Which type of steroid hormone are produced by the kidneys?
Vitamin D3
Which type of steroid hormone are produced by the adrenal cortex?
Corticosteroids
Are steroid hormones lipophilic or hydrophilic?
Lipophilic
->this means they can readily cross the plasma membrane and enter cells which means they have intracellular receptors
What happens when there is activation of intracellular receptors by steroid hormones?
Changes in gene expression which in turns affects protein synthesis
There is a small amount of unbound/ free steroid/thyroid hormone.
What can this hormone do that bound hormone cannot?
Cross capillary walls to target cells.
->amount of free hormone is a lot less than the amount of hormone bound to protein
In order to calculate total plasma hormone levels, what do you need to do?
Free hormone = complex (bound) hormone
Thyroid hormones act like steroid hormones but are actually which type of hormone?
Amine hormones
Are thyroid hormones or lipophilic or hydrophilic?
Lipophilic
->act like steroid hormones
The vast majority of hormone secretion is controlled by which mechanism?
Negative feedback
Neuroendocrine pathways also respond to which other type of feedback as well as negative feedback?
Neural feedback loops
