Introduction to the Endocrine System Flashcards by Zain Aslam

Is the endocrine system anatomically continuous?

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Give an example of fast and slow endocrine responses.

Fast - Increase in HR due to adrenaline

Slow - Increase in protein synthesis die to Growth Hormone

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Give some sites of principle endocrine glands.

Hypothalamus
Pituitary
Thyroid
Parathyroid
Adrenal glands

Renal medulla, renal cortex and in the kidney’s themselves
GI tract
Ovary and testes

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What are the features of an endocrine hormone?

Produced by a cell/group of cells where is secretes into the blood where it is transported to different targets by binding to target receptors

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Do endocrine hormones exert effects at high or low concentrations?

Low - range if 10^-9M to 10^-12M

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What often terminates the endocrine hormones actions?

Neg. feedback loops

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What are some other chemicals that endocrine hormones should NOT be confused with?

Paracrine

Autocrine

Exocrine

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What do paracrine hormones/chemicals do? Give an example of one.

They act local to the site of synthesis rather than travel to distal sites like endocrine does

Histamine

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What do autocrine hormones do?

Example?

They act in/on the same cell that synthesised it

Cytokines

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What do exocrine chems do?

Example?

Released from exocrine glands via ducts to the external environment (including the GI tract as it is classed as external)

Saliva
Sweat
Bile

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What does the endocrine system do in conjunction with the nervous system?

Communicates with and controls all body functions

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What is the difference between an endocrine hormone and a neurotransmitter?

A neurotransmitter acts on the post-synaptic cell to influence its activity but this act is local and not distal

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What is a neuroendocrine system?

NS and endocrine combines - nerves rleases hormones which enter blood and travel to target cells

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Example of a neuroendocrine system?

Hypothalamus -posterior pituitary axis

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How specific is the response of the target cell to hormones?

Very - if the target cell doesn’t have the right receptor, hormones won’t act on it

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Can hormones act on different cells with different effects?

Yes

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Example of a hormone acting on different cells with different effects?

Insulin acts on skeletal muscle/adipose tissue to increase glucose uptake

But acts on liver to increase glycogenesis (make glycogen) and decrease gluconeogenesis

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What is gluconeogenesis?

Synthesising glucose from non-carb substrates like amino-acids

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What is glycogenolysis?

Breaking down glycogen to form glucose

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What are the 3 types of hormone?

Peptide
Steroid
Amine

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Give examples of a peptide hormone?

Insulin
Thyrotropin-releasing hormone
Follicle stimulating hormone

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What makes up a peptide hormone?

Amino acids

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Which type of hormone is most common?

Peptide

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Describe the sysnthesis of a peptide hormone.

An inactive preprohormone is made by ribosomes

Preprohormones have 1 or more copies of the active hormone in the structure, it is then cleaves by the rough ER into a prohormone(s) and is still inactive

Prohormones are then packed into vesicles in the golgi apparatus along with proteolytic enzymes that break the prohormone down into an active hormone and other fragments

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Once the preprohormone becomes the hormone, what happens?

The hormone and fragments are stored in vesicles til needed - when needed entire vesicle contents emptied into blood

What clinical relevance do the fragments have?

Measuring the inactive fragments in plasma is useful to get the exact amount of hormone the body is making Example - for diabetics who take insulin, measuring insulin in blood is not an accurate way of determining how much insulin the body make, but measuring the fragments - C-peptide - is an accurate way

Is there a 1:1 ratio between c-peptide and synthesised insulin?

No since insulin is metabolised faster it is actually 5x higher than c-pep

When the preprohormone become a prohormone - what is the part that is cleaved off called and what is it's job?

Called a signal sequence - it directs the chain to the endoplasmic reticulum

Are peptide hormones hydrophobic/lipophillic or hydrophillic/lipophobic?

Hydrophilic/lipophobic

How do they travel in blood?

Since hydrophilic they do not need carriers and just dissolve in blood

If peptide hormones are lipophobic, what does this mean for its mechanism of action?

They can't cross cell membranes, so must bind to target receptors on the surface of the membrane

Do peptide hormones have a generally fast or slow response once bound?

Fast

What are the 2 pathways used to cause a change in the cell once hormone is bound? What do these pathways cause?

G-protein coupled receptors Tyrosine kinase linked signalling pathways Both pathways phosphorylate proteins in cell causing a change

Explain the G-protein CR pathway

Once hormone binds - activates 2nd messenger system (a cascade) and/or opening/closing of ion channels This leads to modification of proteins and a rapid response

Explain the Tyrosine kinase linked receptor

Alters gene expression of proteins either activating or de-activating the gene Is a slower response time

Can the GPCR pathway also alter gene expression? If so, how?

Yes, the 2nd messengers can alter gene expression

What is an issue with peptide hormones?

They have a short half life and are vulnerable to degradation - must be continuous secreted to have an effect

What are steroid hormones derived from?

Cholesterol

How are different steroid hormones made if they are all derived from cholesterol?

The type of steroid hormone made depends on the fact that different cells have different enzymes synthesizing differing derivatives of cholesterol

Are steroid hormones stored?

No only made as needed

Why are steroid hormones not stored?

They are hydrophobic/lipophillic so can't be kept within the cell membrane

How do they travel in blood if they are hydrophobic?

Carriers such as albumin

What can albumin do for the hormone?

Increase half-life of it

What hormones does the kidney make?

Kidneys make Vit. D3

Mechanism of action for steroid hormones?

Since they are lipophilic, receptors are inside cell They alter gene expression - activation or repression which alters protein sysnthesis

Steroid hormones have a slow or fast response?

Slow when they bind inside cell Some evidence suggests they can bind on membrane for a rapid effect

What are amine hormones derived from?

2 amino acids

What are the 2 amino acids that amine derives from?

Tyrosine | Tryptophan

Which of the 2 amino acids makes all but 1 amine hormone?

Tyrosine

What amine hormone does tryptophan make?

Melatonin

What does melatonin do?

Regulates circadian rhythm

What are the 2 subcatagories of amine hormone that tyrosine makes?

Catacholamines | Thyroid hormones

Name some catacholamines and where they are made

Dopamine - brain Noraadrenaline - neurons Adrenaline - adrenal gland

Mechanism of action for catacholamines?

They are hydrophilic/lipophobic so have same mechanism as peptide hormones

Are catacholamines vulnerable to degradation?

Yes - also need to be continually secreted

Name some thyroid hormones.

Thyroxine | Triiodothyronine

Mechanism of action for thyroid hormones?

Hydrophobic/lipophilic so same as steroid

Why is it important that there is a small amount of unbound free steroid/thyroid hormone in plasma at any given time?

Only unbound can cross membrane So a ratio exists between bound and unbound and is constantly maintained. Example - free hormone degrades due to short half life or binds to receptor = bound hormone to unbind = more homrone produced to replace bound hormone lost

What is the law of mass action?

As free hormone leaves the plasma more hormone is released from carriers

Bound:unbound ratio favours what side?

Bound is higher

In health does the level of free hormones remain constant or vary?

Constant

How much hormone is needed for a repsone?

Minute amounts

How is total plasma hormone conc calculated?

Free hormone + complex bound hormone

What type of hormone carriers are there?

Specific - corticosteroid binding globulin (only binds to one hormone) Non-specific - albumin

How are hormones removed from body?

Excreted or metabolically transformed via liver and kidneys

What hormones are excreted easily?

Peptide and catacholamines since short half life

What hormones take longer to excrete?

Steroids and thyroid hormones since they are protein bound

What are 2 ways hormone secretion is controlled?

Neg feedback loops - high levels of hormone feeds back and stops production Neural feedback loops - sympathetic response causes adrenaline secretion

What happens when there is a prolonged exposure of low hormone conc. on a cell?

The cell will make more receptors to increases sensitivity to the hormone

What happens when there is a prolonged exposure of high hormone conc. on a cell?

The cell decrease its receptors which in turn decreses sensitivity to the hormone

This change in tissue sensitivity can affect receptors for other hormones too - T or F

True - Can have permissive or antagonistic effects

Explain a permissive effect

The presence of one hormone enhances the effect of another For example adrenaline causes a lower rate of lipolysis in adipose tissue than when adrenaline and thyroid hormone are combined = greater lipolysis This is because the presence of thyroid hormone causes cell to make more adrenaline receptors - thyroid hormone has no effect on lipolysis but is permissive to adrenaline

Hormones are released in short bursts and conc. varies widely. Why is this clinically relevant?

A single reading may be misleading - need to monitor over 24 hrs

Look at full hormone table on slide