Introduction to endocrine system

Question 1

what is the definition of the Endocrine System?

Answer 1

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, where they influence the activity of that target organ.

Question 2

how can the response of the endocrine system differ?

Answer 2

Response may be fast (within seconds) e.g. increased heart rate in response to adrenalin, or slow (over days) e.g. increased protein synthesis in response to growth hormone.

Question 3

what does the endocrine system act in conjunction with to control all body functions?

Answer 3

nervous system

Question 4

what is endocrine communication?

Answer 4

Hormones travel in the blood to their target organs/tissues.

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

No receptor = no response

Question 5

what is neural communication?

Answer 5

Neurotransmitters released from presynaptic neurons travel across the synaptic cleft to the postsynaptic cell to influence its activity.

Neurotransmitter is the chemical released by the neuron but, in contrast to hormones, acts locally within the synaptic cleft.

Question 6

what is neuroendocrine?

Answer 6

endocrine and nervous systems combine. Nerves release hormones which enter blood and travel to their target cells e.g. hypothalamic – posterior pituitary axis.

Question 7

what is the specific response from insulin acting on skeletal muscle and liver?

Answer 7

skeletal muscle - increase glucose uotaje

liver - increased glycogenesis, decreased gluconeogenesis

Question 9

what should ductless endocrine glands not be confused with?

Answer 9

Ducted Exocrine glands and secretions
Autocrine secretions
Paracrine secretions

Question 10

what are Paracrine secretions?

Answer 10

chemicals diffuse in ECF to affect neighbouring cells e.g. histamine.

Question 11

what are Autocrine secretions?

Answer 11

cells secrete chemicals that bind to receptors on the same cell e.g. cytokines.

Question 12

what are ducted exocrine glands and secretions?

Answer 12

These glands deliver their secretions to the external environment including the GI tract e.g. bile, saliva and sweat.

Question 13

what is the function of hormones and neurotransmitters?

Answer 13

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

Question 14

what physiological systems have endocrine functions?

Answer 14

reproductive, renal, gastrointestinal

Question 15

what diffuse systems have primarily endocrine functions?

Answer 15

thyroid gland, adrenal gland, pituitary gland, hypothalamus

Question 16

what is an endocrine hormone produced by?

Answer 16

a cell or group of cells

Question 17

what is an endocrine hormone secreted by?

Answer 17

from those cells into the blood

Question 18

what are endocrine hormones transported via?

Answer 18

the blood to distant targets

Question 19

at what concentration do endocrine hormones exert their effects?

Answer 19

at very low concentrations (act in the range 10-9 -10-12 M)

Question 20

how do endocrine hormones act?

Answer 20

by binding to receptors on target tissues - no effect without receptor

Question 21

how is the action of an endocrine hormone often terminated?

Answer 21

via negative feedback loops.

Question 22

how frequently are endocrine hormones released?

Answer 22

Often released in short bursts, meaning 24 hr monitoring is clinically required to get realistic picture

Question 23

what are peptide/protein hormones composed of?

Answer 23

composed of chains of amino acids (most common)

Question 24

what are amine hormones derived from?

Answer 24

all derived from one of two amino acids (tryptophan or tyrosine)

Question 25

what are steroid hormones derived from?

Answer 25

all derived from cholesterol

Question 26

what are the three types of endocrine hormones?

Answer 26

peptide/ protein hormines

amine hormones

steroid hormones

Question 27

what are examples of peptide hormones?

Answer 27

TRH, FSH, insulin

Question 28

how are peptide hormones synthesised?

Answer 28

as preprohormone in advance of need then cleaved into prohormone and stored in vesicles until required.

Question 29

where are peptide prohormones stored?

Answer 29

in vesicles until required.

Question 30

what is a preprohormone?

Answer 30

The initial peptide hormone produced by ribosomes

large and inactive

contain one or more copies of the active hormone in their amino acid sequence as well as a signal sequence.

Question 31

what is a prohormone?

Answer 31

The signal sequence is cleaved off in the endoplasmic reticulum to leave the smaller but still inactive protein

Question 32

how are prohormones packaged into vesicles?

Answer 32

s in the golgi apparatus, along with proteolytic enzymes which break the prohormone down into active hormone and other fragments.

Hormones and fragments are stored in vesicles in the endocrine cells until release is triggered then all vesicle contents are released into plasma (co-secretion).

Question 33

what is c-peptide?

Answer 33

inactive fragment cleaved from the insulin prohormone

Question 34

what is the clinical significance of c-peptide?

Answer 34

Levels of C-peptide in plasma or urine are often measured to indicate endogenous insulin production from the pancreas (produced in equal amounts).

Question 35

Why measure C-peptide and not insulin directly?

Answer 35

can measure insulin but prefer not to
if you measure insulin it also measures the insulin tht they are injecting not just the function of the pancreas

Question 36

are levels of insulin or c-peptide higher?

Answer 36

because insulin is metabolised faster, levels of C-peptide are typically about 5x higher than endogenous insulin.

Question 37

describe the mechanism of action of peptide hormones?

Answer 37

Water soluble so dissolve easily in plasma making transport via the blood simple and easy. Water solubility however means cannot cross cell membrane so bind to membrane bound receptors on target cell.

Question 38

what are the two types of receptors peptide hormones targert?

Answer 38

either GPCR or tyrosine kinase linked receptors

Question 39

are peptide hormones hydrophilic or hydrophobic?

Answer 39

hydrophylic

Question 40

how does a g coupled receptor react following binding?

Answer 40

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

Question 41

how does a Tyrosine Kinase Linked Receptor react following binding?

Answer 41

Alters gene expression*. Slower, longer lasting activity (mintues / hours / days)

Question 42

what amino acids are most amine hormones derived from?

Answer 42

amino acid tyrosine

Question 43

what are examples of amine hormones?

Answer 43

Dopamine (CNS neurotransmitter)

Norepinephrine (neurotransmitter)

Epinephrine (hormone released by adrenal medulla)

Question 44

what is the only amine hormone not derived from tyrosine?

Answer 44

melatonin

Question 45

what is melatonin derived from?

Answer 45

tryptophan

Question 46

are amine hormones hydrophilic or hydrophobic?

Answer 46

hydrophylic

Question 47

are steroid hormones hydrophilic or hydrophobic?

Answer 47

hydrophobic / highly lipophilic

Question 48

describe the mechanism of action of steroid hormones?

Answer 48

steroid hormones are synthesized directly as needed, rather than being stored and released. This is because they are highly lipophilic (lipid soluble) so cannot be retained within lipid membranes. Once synthesized they diffuse across the membrane into the ISF and the blood.
Being poorly soluble in water they are transported bound to carrier proteins such as albumin. This stabilises their transport through the plasma and protects them from enzymatic degradation.

Lipophilicity allows steroid hormones to readily cross the plasma membrane, both into and out of cells. As such their receptors are located inside cells (cytoplasmic or nuclear receptors).

Activation of the intracellular receptors leads to a change in gene expression at the level of the nucleus = genomic effect.

Genes control the synthesis of protein so these hormones either ↑ or ↓ protein synthesis.

This is a relatively 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.

Question 49

what is the half life of steroid hormones versus amine hormones

Answer 49

Half life 60-90mins (vs 2mins for amine hormones)

Question 50

what is the onset of action for steroid hormones?

Answer 50

Slow onset of action but action persists much longer than that of peptide or amine hormones.

Question 51

what are steroid hormones produced by?

Answer 51

Gonads (testes and ovary) – sex steroids

Placenta - hCG (measures in pregnacy test), sex steroids

Kidney - Vitamin D3

Adrenal cortex - corticosteriods

Question 52

describe the physiological activity of lipophilic hormones?

Answer 52

There is a small amount of unbound free steroid/thyroid hormone in the plasma and this is the physiologically important fraction.
Only free hormone can diffuse across capillary walls to target cells.
Free hormone : protein bound hormone ratio much in favour of bound (complexed) hormone
The extent of protein binding can have important effects on the hormone’s actions but in health levels of free (active) hormone remain constant. The Law of Mass Action dictates that as free hormone leaves the plasma (taken up by cells) more hormone is released from the carriers. Typically only minute quantities of hormone are required for physiological functions.
Free hormone + complexed hormone = Total plasma [hormone]

Question 53

summarise peptide and catecholamine hormones?

Answer 53

are water soluble, transported in solution in the plasma.
are vulnerable to degradation before they reach their target.
have a short half-life in the plasma, (time taken for [plasma] concentration to fall by a half),usually minutes. Prolonged action therefore requires continued secretion.

Question 54

summarise steroid and thyroid hormones?

Answer 54

are lipophilic, once made they diffuse across the cell membrane into the blood.
circulate in plasma bound to specific transport plasma proteins (eg thryoxine-binding globulin, or albumin) so have longer half-life, usually hours to days.
Alter protein synthesis via modifying gene expression thus effect also persists for hours to days.