Intro to Endocrine system Flashcards
Definition of the Endocrine system
System that integrates and controls organ function via the secretion of chemicals (hormones) from cells, tissues or glands
These are then carried in the blood to target organs, distal from the site of hormone synthesis.
Here, they influence the activity of that target organ.
Where do paracrine chemicals act on? Give example of a paracrine chemical
act local to the site of synthesis, do not travel to distant sites e.g. histamine
diffuse in the ECF to affect neighbouring cells
Where do autocrine chemicals act? Give example of an autocrine chemical
act on/in the same cell that synthesises the hormone e.g. cytokines
don’t travel in the blood
Where are Exocrine chemicals released from? Give examples (3)
Released from exocrine glands via ducts to the external environment including the GI tract e.g. saliva, sweat, bile
How do tissues detect hormones?
through the presence of specific receptors for that chemical on/in the cells.
No receptor = no response
How is neural communication different?
Neurotransmitters are the chemical released by the neuron but, in contrast to hormones, they act locally within the synaptic cleft.
What is the Neuroendocrine system
Endocrine and nervous systems combine.
Nerves release hormones which enter blood (not to synapse) and travel to their target cells
Best example of neuroendocrine system?
Hypothalamic - posterior pituitary axis
Features of an endocrine hormone (6)
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 in the range 10-9 -10-12 M) - very potent in their effect
Act by binding to receptors on target tissues
Have their action terminated, often via negative feedback loops.
What are the 3 types of endocrine hormones?
Peptide or protein hormones – composed of chains of amino acids (most common)
Amine hormones – all derived from one of two amino acids (tryptophan or tyrosine)
Steroid hormones – all derived from cholesterol
Give 3 examples of peptide hormones
Thyrotropin-Releasing Hormone - TRH
Follicle-stimulating hormone - FSH
Insulin
Glucagon
what produces steroid hormones? (4)
Gonads (testes and ovary) – sex steroids
Placenta - hCG, sex steroids
Kidney - Vitamin D3
Adrenal cortex - corticosteriods
What is the response when insulin binds to its target tissue of skeletal muscle/adipose tissue?
increase glucose uptake from the blood
Response when insulin acts on the liver
increased glycogenesis
decreased gluconeogenesis
ie converts glucose into glycogen for storage and stops producing new glucose in order to prevent hyperglycaemia
How are peptide hormones synthesised
preprohormone (inactive hormone that contains one or more copies of the active hormone in their amino acid sequence) peptide chain made in the ribosome.
The chain moves into the ER where enzymes cleave it creating prohormone
Prohormone moves to golgi apparatus where it’s packaged up in vesicles along with enzymes. The enzyme turns the prohormone into active hormone
vesicles are exocytosed when signal reaches endocrine cell
Peptide hormones stored in their tissue until release
True or false: Steroid hormones are produced and released on demand
true
what is c-peptide?
inactive fragment cleaved from the insulin prohormone
What is measured to indicate endogenous insulin production from the pancreas?
c-peptide levels in urine or plasma
levels of c-peptide are typically about 5x higher than endogenous insulin as c-peptide is metabolised slower in the blood
Mechanism of action of peptide hormones
Water soluble - dissolve easily in plasma making transport via the blood simple and easy.
Cannot cross cell membrane so bind to membrane bound receptors on target cell.
Once bound these receptors generally create relatively fast biological responses (seconds to minutes). Most peptide hormones work via modulating either the 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.
Most peptide hormones work by modulating which 2 signalling pathways?
GPCR
Tyrosine kinase linked
How does GPCR pathway work?
Activates 2nd messenger system and/or ion channels leading to modification of existing proteins
Rapid response
How does tyrosine kinase pathway work?
Alters gene expression
Slower, long lasting activity
Which amino acids are most amine hormones derived from?
Tyrosine
Name 3 common amine hormones
Dopamine (CNS neurotransmitter)
Norepinephrine (neurotransmitter)
Epinephrine (adrenaline - hormone released by adrenal medulla)
What is the only amine hormone not derived from tyrosine? what is it’s function
Melatonin - derived from tryptophan
regulates circadian rhythm
What happens when steroid hormones are synthesised
They diffuse across the membrane into the interstitial fluid and the blood.
They are transported through the plasma (95% water) bound to carrier proteins such as albumin - this protects them from enzymatic degradation (this increases their half life)
Slower onset but last for longer
What are all steroid hormones derived from? What problem does this cause?
Cholesterol
Mixed messaging/cross effects when steroids are in excess
Mechanism of action of steroid hormones
Lipophilic - cross the plasma membrane easily, both into and out of cells
only the free hormone is active though so only once it is released from protein carrier can it cross plasma memebrane
Their receptors lie within cells and trigger either activation or inhibition of gene function within the nucleus
genes control the synthesis of protein so these hormones either increase or decrease protein synthesis - there’s a lag time between hormone release and effect so takes time to reverse this too
What does the Law of Mass action dictate?
As free hormone leaves the plasma (taken up by cells) more hormone is released from the carriers.
What is secretion responsive to in most endocrine pathways?
Negative feedback reflexes
What control mechanisms exist that influence [insulin] in plasma? (4)
Plasma [glucose]
Autonomic nerve activity (parasympathetic)
Presence of food in the gut
Plus additional hormones such as glucagon
