Introduction to the Endocrine System Hormones, Receptors and Signalling Flashcards
where does endocrine secretion originate?
either single cells
more commonly collection of cells forming a ductless endocine gland
endocrine vs exocrine secretion?
exocrine = secretes into ducts endocrine = ductless secretion
how do glands communicate with other tissues in the body/other endocrine glands in the body?
via secretion of hormone into the blood and bulk transport to the target site
how do endocrine glands interact? give an example
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
what 3 factors are involved in the specificity of hormone signalling?
chemically distinct hormones
specific receptors for each hormone
distinct distribution of receptors across target cells
what are the 7 major endocrine organs in the body?
pituitary thyroid parathyroid adrenal glands pancreas ovary testes - others do exist - hypothalamus, pineal, heart etc
what classifies an endocrine organ?
secretes a hormone
therefore many organs in the body can technically be considered an endocrine hormone - heart, kidneys etc
what are the 4 main classes of hormones?
modified amino acids
steroids - synthesised from cholesterol
peptides
proteins
give an example of each hormone type
amino acid - adrenaline, thyroid hormones
steroids - cortisol, progesterone, testosterone
peptides - ACTH, ADH, oxytocin
proteins - insulin
peptide vs protein hormones?
peptides derived from larger precursor proteins
significance of hormone structure?
half life
where it acts
what are the 3 types of signalling?
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
do all hormones have a single distinct signalling action?
no
e.g - somatostatin
how potent are hormones?
very potent
only very small concentration required to activate receptors at distant sites
- receptors have high affinity for the hormones
each hormone has a single specific target, true or false?
false
single hormone can act on several targets and several hormones can act on same target
how does a hormone elicit a biological response?
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
how does hormone action differ?
speed of onset
duration of action
- most are slower and more long lasting compared to neurotransmitters
how is hormone action terminated?
enzyme mediated metabolic inactivation in either the liver or site of action
what 2 broad actions can a hormone have?
complementary
antagonistic
describe a complementary action
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
describe antagonistic action
occurs via balance of opposing influences
- e.g insulin lowers plasma glucose while glucagon increases plasma glucose
describe the lifecycle of amine hormones
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
describe the life cycle of peptides and proteins hormones
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
describe the life cycle of steroid hormones
synthesised from cholesterol upon demand
released immediately once formed
transported in plasma bound to plasma transport proteins (90%) as hydrophobic
how are steroid hormones synthesised?
stimuli increases cellular uptake and availability of cholesterol and rate of conversion of cholesterol to pregnenolone
(all steroids hormones formed via pregnenolone intermediate)
which part of steroid hormone is biologically active?
only free, unbound hormone in the blood
which hormones are insoluble in plasma? how are they transported?
steroids
thyroid hormones
mainly bound to carrier proteins
what are the functions of carrier proteins?
increase amount of insoluble hormones in blood
provide reservoir of hormone
extend half life of hormone in circulation
name 3 carrier proteins and their specific functions
cortisol binding globulin
thyroxine binding globulin
sex steroid binding globulin
name 2 general carrier proteins
albumin
transthyretin
- bind several hormones
how do carrier proteins help maintain constant concentration of hormone in the blood?
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
what is the primary determinant of plasma concentration?
rate of secretion
how is secretion of hormone generally controlled, give an example?
negative feedback - eg HPA axis
*see diagram in lecture
give an example of how the nervous system and endocrine system can interact
HPA axis
neurological stress can trigger hypothalamus to produce CHF etc
what is the general pattern of hormone secretion (e.g cortisol)?
diurinal (circadium) rhythm
- secretion rate goes up and down as a function of time (entrained to external cue - i.e night and day)
via what routes are hormones usually eliminated?
metabolism locally]
by the liver
excretion by the liver
how can plasma concentration of hormone be calculated?
rate of secretion - rate of elimination
what are the major types of hormone receptors?
g protein coupled receptors (cell surface) receptor kinases (cell surface) nuclear receptors (intracellular)
what activates GPCRs?
amines and some proteins/peptides
signalling pathways involve coupling to Gs, Gi or Gg
what activates receptor kinases?
some proteins/peptides
what are the 3 classes of nuclear receptors?
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
what are the general effects of Gs and Gi?
Gs increases adenylyl cyclase activity = increases cAMP = increased PKA = cellular effects
Gi = suppresses adenylyl cyclase activity etc
what happens in receptor kinases when insulin binds
beta subunits gain enzyme activity (by increasing tyrosine kinase activity) so can facilitate phosphorylation of tyrosine > IRS1 binds > IRS1 phosphorylated > activates PKB > metabolic effects
