Introduction to the Endocrine System Hormones, Receptors and Signalling

Question 1

where does endocrine secretion originate?

Answer 1

either single cells

more commonly collection of cells forming a ductless endocine gland

Question 2

endocrine vs exocrine secretion?

Answer 2

exocrine = secretes into ducts
endocrine = ductless secretion

Question 3

how do glands communicate with other tissues in the body/other endocrine glands in the body?

Answer 3

via secretion of hormone into the blood and bulk transport to the target site

Question 4

how do endocrine glands interact? give an example

Answer 4

glands are anatomically distinct strucures but form integrated functional systems
- e.g hypothalamus releases CRF which stimulates anterior pituitary to release ACTH which causes adrenal cortex to synthesise and release cortisol

Question 5

what 3 factors are involved in the specificity of hormone signalling?

Answer 5

chemically distinct hormones
specific receptors for each hormone
distinct distribution of receptors across target cells

Question 6

what are the 7 major endocrine organs in the body?

Answer 6

pituitary
thyroid
parathyroid
adrenal glands
pancreas
ovary
testes
- others do exist - hypothalamus, pineal, heart etc

Question 7

what classifies an endocrine organ?

Answer 7

secretes a hormone

therefore many organs in the body can technically be considered an endocrine hormone - heart, kidneys etc

Question 8

what are the 4 main classes of hormones?

Answer 8

modified amino acids
steroids - synthesised from cholesterol
peptides
proteins

Question 9

give an example of each hormone type

Answer 9

amino acid - adrenaline, thyroid hormones
steroids - cortisol, progesterone, testosterone
peptides - ACTH, ADH, oxytocin
proteins - insulin

Question 10

peptide vs protein hormones?

Answer 10

peptides derived from larger precursor proteins

Question 11

significance of hormone structure?

Answer 11

half life

where it acts

Question 12

what are the 3 types of signalling?

Answer 12

autocrine - hormone released from cell into ECM and acts back upon itself
paracrine - hormone released from cell and acts on cells immediately adjacent (e.g histamine in stomach)
endocrine - hormone released into ECM and enters the blood stream and travels to target cells and leaves the blood

Question 13

do all hormones have a single distinct signalling action?

Answer 13

no

e.g - somatostatin

Question 14

how potent are hormones?

Answer 14

very potent
only very small concentration required to activate receptors at distant sites
- receptors have high affinity for the hormones

Question 15

each hormone has a single specific target, true or false?

Answer 15

false

single hormone can act on several targets and several hormones can act on same target

Question 16

how does a hormone elicit a biological response?

Answer 16

hormone binds to its cognate receptor which is expressed in a tissue selective manner
activated receptor engages a signal transduction cascade that differs between individual receptors but typically causes amplification of the original signal. This is important because of the scarcity of the original signal

Question 17

how does hormone action differ?

Answer 17

speed of onset
duration of action
- most are slower and more long lasting compared to neurotransmitters

Question 18

how is hormone action terminated?

Answer 18

enzyme mediated metabolic inactivation in either the liver or site of action

Question 19

what 2 broad actions can a hormone have?

Answer 19

complementary

antagonistic

Question 20

describe a complementary action

Answer 20

action of several hormones regulates many complex physiological functions on short and long time scales
- e.g adrenaline, cortisol and glucagon contribute to body’s response to exercise

Question 21

describe antagonistic action

Answer 21

occurs via balance of opposing influences

- e.g insulin lowers plasma glucose while glucagon increases plasma glucose

Question 22

describe the lifecycle of amine hormones

Answer 22

presynthesised via enzymatic steps from tyrosine
stored in vesicles in endocrine cell
released into ECM via C2+ dependant exocytosis in response to stimuli
transported free in the plasma (dissolved in blood) as amines are hydrophillic

Question 23

describe the life cycle of peptides and proteins hormones

Answer 23

presynthesised from long precursor protein via proteolytic steps
mature hormones stored in vesicles in endocrine cells
released into ECM via Ca2+ dependant exocytosis in response to stimuli
transported free in the blood plasma like amines

Question 24

describe the life cycle of steroid hormones

Answer 24

synthesised from cholesterol upon demand
released immediately once formed
transported in plasma bound to plasma transport proteins (90%) as hydrophobic

Question 25

how are steroid hormones synthesised?

Answer 25

stimuli increases cellular uptake and availability of cholesterol and rate of conversion of cholesterol to pregnenolone
(all steroids hormones formed via pregnenolone intermediate)

Question 26

which part of steroid hormone is biologically active?

Answer 26

only free, unbound hormone in the blood

Question 27

which hormones are insoluble in plasma? how are they transported?

Answer 27

steroids
thyroid hormones
mainly bound to carrier proteins

Question 28

what are the functions of carrier proteins?

Answer 28

increase amount of insoluble hormones in blood
provide reservoir of hormone
extend half life of hormone in circulation

Question 29

name 3 carrier proteins and their specific functions

Answer 29

cortisol binding globulin
thyroxine binding globulin
sex steroid binding globulin

Question 30

name 2 general carrier proteins

Answer 30

albumin
transthyretin
- bind several hormones

Question 31

how do carrier proteins help maintain constant concentration of hormone in the blood?

Answer 31

bound hormones are too large to cross capillary wall
allows equilibrium between free and bound equilibrium as hormone can unbind and replace free hormone which has crossed the capillary wall - therefore free concentration of hormone doesn’t rise abruptly during a surge of hormone secretion

Question 32

what is the primary determinant of plasma concentration?

Answer 32

rate of secretion

Question 33

how is secretion of hormone generally controlled, give an example?

Answer 33

negative feedback - eg HPA axis

*see diagram in lecture

Question 34

give an example of how the nervous system and endocrine system can interact

Answer 34

HPA axis

neurological stress can trigger hypothalamus to produce CHF etc

Question 35

what is the general pattern of hormone secretion (e.g cortisol)?

Answer 35

diurinal (circadium) rhythm

- secretion rate goes up and down as a function of time (entrained to external cue - i.e night and day)

Question 36

via what routes are hormones usually eliminated?

Answer 36

metabolism locally]
by the liver
excretion by the liver

Question 37

how can plasma concentration of hormone be calculated?

Answer 37

rate of secretion - rate of elimination

Question 38

what are the major types of hormone receptors?

Answer 38

g protein coupled receptors (cell surface)
receptor kinases (cell surface)
nuclear receptors (intracellular)

Question 39

what activates GPCRs?

Answer 39

amines and some proteins/peptides

signalling pathways involve coupling to Gs, Gi or Gg

Question 40

what activates receptor kinases?

Answer 40

some proteins/peptides

Question 41

what are the 3 classes of nuclear receptors?

Answer 41

class 1 = in cytoplasm bound to HSP and move to nucleus when activated by steroid hormones
class 2 = in nucleus, activated by lipids
hybrid = activated by thyroid hormones, similar action to class 1

Question 42

what are the general effects of Gs and Gi?

Answer 42

Gs increases adenylyl cyclase activity = increases cAMP = increased PKA = cellular effects
Gi = suppresses adenylyl cyclase activity etc

Question 43

what happens in receptor kinases when insulin binds

Answer 43

beta subunits gain enzyme activity (by increasing tyrosine kinase activity) so can facilitate phosphorylation of tyrosine > IRS1 binds > IRS1 phosphorylated > activates PKB > metabolic effects