Intro to Endocrine system

Question 1

Definition of the Endocrine system

Answer 1

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.

Question 2

Where do paracrine chemicals act on? Give example of a paracrine chemical

Answer 2

act local to the site of synthesis, do not travel to distant sites e.g. histamine

diffuse in the ECF to affect neighbouring cells

Question 3

Where do autocrine chemicals act? Give example of an autocrine chemical

Answer 3

act on/in the same cell that synthesises the hormone e.g. cytokines

don’t travel in the blood

Question 4

Where are Exocrine chemicals released from? 
Give examples (3)

Answer 4

Released from exocrine glands via ducts to the external environment including the GI tract e.g. saliva, sweat, bile

Question 5

How do tissues detect hormones?

Answer 5

through the presence of specific receptors for that chemical on/in the cells.

No receptor = no response

Question 6

How is neural communication different?

Answer 6

Neurotransmitters are the chemical released by the neuron but, in contrast to hormones, they act locally within the synaptic cleft.

Question 7

What is the Neuroendocrine system

Answer 7

Endocrine and nervous systems combine.

Nerves release hormones which enter blood (not to synapse) and travel to their target cells

Question 8

Best example of neuroendocrine system?

Answer 8

Hypothalamic - posterior pituitary axis

Question 9

Features of an endocrine hormone (6)

Answer 9

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.

Question 10

What are the 3 types of endocrine hormones?

Answer 10

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

Question 11

Give 3 examples of peptide hormones

Answer 11

Thyrotropin-Releasing Hormone - TRH
Follicle-stimulating hormone - FSH
Insulin
Glucagon

Question 12

what produces steroid hormones? (4)

Answer 12

Gonads (testes and ovary) – sex steroids

Placenta - hCG, sex steroids

Kidney - Vitamin D3

Adrenal cortex - corticosteriods

Question 13

What is the response when insulin binds to its target tissue of skeletal muscle/adipose tissue?

Answer 13

increase glucose uptake from the blood

Question 14

Response when insulin acts on the liver

Answer 14

increased glycogenesis
decreased gluconeogenesis

ie converts glucose into glycogen for storage and stops producing new glucose in order to prevent hyperglycaemia

Question 15

How are peptide hormones synthesised

Answer 15

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

Question 16

True or false: Steroid hormones are produced and released on demand

Answer 16

true

Question 17

what is c-peptide?

Answer 17

inactive fragment cleaved from the insulin prohormone

Question 18

What is measured to indicate endogenous insulin production from the pancreas?

Answer 18

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

Question 19

Mechanism of action of peptide hormones

Answer 19

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.

Question 20

Most peptide hormones work by modulating which 2 signalling pathways?

Answer 20

GPCR

Tyrosine kinase linked

Question 21

How does GPCR pathway work?

Answer 21

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

Rapid response

Question 22

How does tyrosine kinase pathway work?

Answer 22

Alters gene expression

Slower, long lasting activity

Question 23

Which amino acids are most amine hormones derived from?

Answer 23

Tyrosine

Question 24

Name 3 common amine hormones

Answer 24

Dopamine (CNS neurotransmitter)

Norepinephrine (neurotransmitter)

Epinephrine (adrenaline - hormone released by adrenal medulla)

Question 25

What is the only amine hormone not derived from tyrosine? what is it’s function

Answer 25

Melatonin - derived from tryptophan

regulates circadian rhythm

Question 26

What happens when steroid hormones are synthesised

Answer 26

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

Question 27

What are all steroid hormones derived from? What problem does this cause?

Answer 27

Cholesterol

Mixed messaging/cross effects when steroids are in excess

Question 28

Mechanism of action of steroid hormones

Answer 28

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

Question 29

What does the Law of Mass action dictate?

Answer 29

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

Question 30

What is secretion responsive to in most endocrine pathways?

Answer 30

Negative feedback reflexes

Question 31

What control mechanisms exist that influence [insulin] in plasma? (4)

Answer 31

Plasma [glucose]

Autonomic nerve activity (parasympathetic)

Presence of food in the gut

Plus additional hormones such as glucagon