Endocrine intro Flashcards by Heo Mun Ho

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

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Define a neurohormone

any hormone produced and released by neuroendocrine cells into the blood

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Define a neurohormone

any hormone produced and released by neuroendocrine cells into the blood

once in the blood, acts like any other endocrine hormone

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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

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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

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Endocrine system works together with what system to control all body functions

Nervous

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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)

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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

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Classify hormones according to their chemistry (3)

Peptide/protein hormones (most common type)
Amine hormones
Steroid hormones

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Basic function of hormones

bring about changes in the activity of their target cells and tissues, (increase/decrease a particular activity

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What glands are part of the endocrine system (7)

pineal gland
thyroid gland,
parathyroid glands
adrenal glands,
pituitary gland, 
hypothalamus,
pancreas

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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

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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

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What are peptide hormones composed of

chains of amino acids

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What 2 amino acids are amine hormones derived from

Tryptophan (only 1 amine hormone derived from this) or tyrosine (majority derived from this)

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What are steroid hormones derived from

Cholesterol

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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

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Define a preprohormone

initial peptide hormone produced by ribosomes that’s large and inactive

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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

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What inactive fragment can clinically useful to measure in the plasma in diabetes

C-peptide

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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

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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