Introduction to Endocrinology

Question 1

Define endocrine system

Answer 1

A system that integrates + controls organ function via secretion of hormones which are carried in the blood and which they influence the activity of the target organ

Question 2

Give an example where the body responds quickly to a hormone being released

Answer 2

Increased heart rate in response to adrenaline

Question 3

Give an example where the body responds slower to a hormone being released

Answer 3

Increased protein synthesis in response to growth hormone

Question 4

What are paracrine chemicals?

Answer 4

Chemicals that act locally at the site of synthesis, e.g. histamine

Question 5

What are autocrine chemicals?

Answer 5

Chemicals that act on the same cell that synthesises the hormone, e.g. cytokines

Question 6

What are exocrine chemicals?

Answer 6

Chemicals released from an exocrine gland via ducts to the external environment (incl. GI tract)

Question 7

How do tissues detect hormones?

Answer 7

Via presence of receptors for that chemical in or on the cell (basically no receptor = no response)

Question 8

What is involved in neural communication?

Answer 8

Neurotransmitters act over a synaptic cleft to influence the activity of the post-synaptic cell

Question 9

What is the neuroendocrine system?

Answer 9

Endocrine + nervous systems combined

Nerves release hormones which enter the blood

Question 10

Why do hormones only act on specific target cells despite them being distributed in the blood?

Answer 10

Only the specific target cells have the receptors for any specific hormone

Question 11

Does 1 hormone have the exact same action on different tissues with the receptor for the hormone?

Answer 11

No - the same hormone can have different effects in different target cells

Question 12

What is the function of hormones?

Answer 12

To bring about changes in the activities of their target cells/tissues

Question 13

How are endocrine hormone levels controlled?

Answer 13

Via negative feedback loops

Some endocrine pathways also respond to neural feedback loops

Question 14

What are the three classifications of endocrine hormones?

Answer 14

Peptide/protein
Steroid
Amine

Question 15

What are peptide/protein hormones composed of?

Answer 15

Amino acid chains

Question 16

What are steroid hormones derived from?

Answer 16

Cholesterol

Question 17

What are amine hormones derived from?

Answer 17

One of two amino acids (tryptophan or tyrosine (most common))

Question 18

Give examples of peptide hormones

Answer 18

TRH, FSH, insulin

Question 19

Describe how peptide hormones are made

Answer 19

Synthesised in advance of need + stored in vesicles

Initial hormone produced by ribosome is v. large (preprohormone) + may have 1+ copies of the active hormone

Preprohormone cleaved in RER into smaller, (still) inactive proteins - prohormones

Prohormones packaged in vesicles in Golgi apparatus with proteolytic enzymes which break it down into active hormone + other fragments which are stored in vesicles until release is triggered and all contents are released into the plasma (co-secretion)

Question 20

In which situation may measuring inactive fragments in plasma be useful?

Answer 20

C-peptide in diabetes to check for endogenous insulin production

Question 21

What is C-peptide?

Answer 21

Inactive fragment cleaved from insulin prohormone

Question 22

What is the ratio of production of C-peptide to insulin?

Answer 22

1:1

Insulin metabolised faster so levels of C-peptide 5x higher

Question 23

How do peptide hormones travel in the blood?

Answer 23

Water soluble so easily dissolve in plasma

Question 24

How do peptide hormones act on their target cells?

Answer 24

Bind to membrane receptors on target cell as cannot cross cell membrane

Question 25

How do most peptide hormones work?

Answer 25

Via modulating either GPCR or tyrosine kinase linked signalling pathways

Question 26

How do GPCR (G protein couple receptor) and tyrosine kinase linked signalling pathways work?

Answer 26

Phosphorylate existing proteins in the cell + modify their function, e.g. inactivating/activating enzymes/opening ion channels etc.

Question 27

How fast do peptide hormones cause changes in the cell after they bind to their membrane receptor?

Answer 27

In seconds to minutes

Question 28

How do GPCRs work?

Answer 28

Activates 2nd messenger system and/or ion channels leading to modification of existing proteins

2nd messengers may also alter gene expression

Question 29

How do tyrosine kinase linked receptors work?

Answer 29

Alter gene expression (so slower, but longer lasting activity)

Question 30

How are steroid hormones produced?

Answer 30

Produced as req.

Diffuse across membrane into ISF and blood

Question 31

Why are steroid hormones produced as required?

Answer 31

They are so lipophilic that they could not be retained by a lipid membrane

Question 32

How are steroid hormones transported in the blood?

Answer 32

Bound to carrier proteins, e.g. albumin (this protects them from enzymatic degradation and hence they have a long half life)

Question 33

Name 4 organs that produce steroid hormones

Answer 33

Gonads - sex hormones
Placenta - hCG, sex steroids
Kidney - vitamin D3
Adrenal cortex - corticosteroids

Question 34

What is the mechanism of action of steroid hormones?

Answer 34

Lipophilic + so cross plasma membrane, so receptors are inside cells (cytoplasmic/nuclear) and trigger the activation/repression of gene this leads to increased or decreased protein synthesis

Question 35

What is the activation/repression of gene function by steroid hormones known as?

Answer 35

Genomic effect

Question 36

Give examples of amine hormones

Answer 36

Dopamine, adrenaline, noradrenaline

Question 37

What amine hormone is derived from tryptophan and what is its function?

Answer 37

Melatonin

Regulates circadian rhythm

Question 38

With regards to steroid/thyroid hormone, which proportion is biologically active and can cross capillary walls to enter target cells?

Answer 38

The unbound, free hormone in the plasma

Question 39

True or false:

The free hormone: hormone-protein complex ratio is in favour of free hormone

Answer 39

False

Question 40

What is the Law of Mass Action?

Answer 40

As free hormone leaves the plasma (taken up by cells) more hormone is released from the carriers

Question 41

How do you work out the total plasma hormone concentration?

Answer 41

Free hormone + complexed hormone

Question 42

What does the hormone concentration in the blood depend on?

Answer 42

Rate of secretion vs rate of removal

Question 43

How are hormone removed from the blood?

Answer 43

Excretion/metabolic transformation in liver and kidneys

Question 44

Which hormones are excreted quickly?

Answer 44

Amines and peptides

Question 45

Which hormones are excreted more slowly and why?

Answer 45

Steroid/thyroid hormones as they are protein bound

Question 46

How can hormones influence the ability of target cells to respond?

Answer 46

Regulation of the number of hormone receptors:
1. upregulation (if prolonged exposure to low hormone conc.) which increases tissue sensitivity to hormone

2. downregulation (after prolonged high exposure to hormone) which decreases sensitivity to hormone

Question 47

What is a permissive effect?

Answer 47

Presence of one hormone enhances the effect of another

Question 48

Give an example of a permissive effect

Answer 48

Epinephrine causes modest lipolysis in adipose tissue, but if thyroid hormones present greatly increased lipolysis occurs

TH –> increased synthesis of receptors for epinephrine on adipocytes

Question 49

Why do you often need a 24 hour measurement for hormones?

Answer 49

Most hormones released in short burst so single values can be misleading