Introduction to endocrine system Flashcards
what is the definition of the Endocrine System?
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.
how can the response of the endocrine system differ?
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.
what does the endocrine system act in conjunction with to control all body functions?
nervous system
what is endocrine communication?
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
what is neural communication?
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.
what is neuroendocrine?
endocrine and nervous systems combine. Nerves release hormones which enter blood and travel to their target cells e.g. hypothalamic – posterior pituitary axis.
what is the specific response from insulin acting on skeletal muscle and liver?
skeletal muscle - increase glucose uotaje
liver - increased glycogenesis, decreased gluconeogenesis
what should ductless endocrine glands not be confused with?
Ducted Exocrine glands and secretions
Autocrine secretions
Paracrine secretions
what are Paracrine secretions?
chemicals diffuse in ECF to affect neighbouring cells e.g. histamine.
what are Autocrine secretions?
cells secrete chemicals that bind to receptors on the same cell e.g. cytokines.
what are ducted exocrine glands and secretions?
These glands deliver their secretions to the external environment including the GI tract e.g. bile, saliva and sweat.
what is the function of hormones and neurotransmitters?
bring about changes in the activity of their target cells and tissues, (increase/decrease a particular activity).
what physiological systems have endocrine functions?
reproductive, renal, gastrointestinal
what diffuse systems have primarily endocrine functions?
thyroid gland, adrenal gland, pituitary gland, hypothalamus
what is an endocrine hormone produced by?
a cell or group of cells
what is an endocrine hormone secreted by?
from those cells into the blood
what are endocrine hormones transported via?
the blood to distant targets
at what concentration do endocrine hormones exert their effects?
at very low concentrations (act in the range 10-9 -10-12 M)
how do endocrine hormones act?
by binding to receptors on target tissues - no effect without receptor
how is the action of an endocrine hormone often terminated?
via negative feedback loops.
how frequently are endocrine hormones released?
Often released in short bursts, meaning 24 hr monitoring is clinically required to get realistic picture
what are peptide/protein hormones composed of?
composed of chains of amino acids (most common)
what are amine hormones derived from?
all derived from one of two amino acids (tryptophan or tyrosine)
what are steroid hormones derived from?
all derived from cholesterol
what are the three types of endocrine hormones?
peptide/ protein hormines
amine hormones
steroid hormones
what are examples of peptide hormones?
TRH, FSH, insulin
how are peptide hormones synthesised?
as preprohormone in advance of need then cleaved into prohormone and stored in vesicles until required.
where are peptide prohormones stored?
in vesicles until required.
what is a preprohormone?
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.
what is a prohormone?
The signal sequence is cleaved off in the endoplasmic reticulum to leave the smaller but still inactive protein
how are prohormones packaged into vesicles?
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).
what is c-peptide?
inactive fragment cleaved from the insulin prohormone
what is the clinical significance of c-peptide?
Levels of C-peptide in plasma or urine are often measured to indicate endogenous insulin production from the pancreas (produced in equal amounts).
Why measure C-peptide and not insulin directly?
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
are levels of insulin or c-peptide higher?
because insulin is metabolised faster, levels of C-peptide are typically about 5x higher than endogenous insulin.
describe the mechanism of action of peptide hormones?
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.
what are the two types of receptors peptide hormones targert?
either GPCR or tyrosine kinase linked receptors
are peptide hormones hydrophilic or hydrophobic?
hydrophylic
how does a g coupled receptor react following binding?
Activates 2nd messenger system and/or ion channels leading to modification of existing proteins. Rapid response
how does a Tyrosine Kinase Linked Receptor react following binding?
Alters gene expression*. Slower, longer lasting activity (mintues / hours / days)
what amino acids are most amine hormones derived from?
amino acid tyrosine
what are examples of amine hormones?
Dopamine (CNS neurotransmitter)
Norepinephrine (neurotransmitter)
Epinephrine (hormone released by adrenal medulla)
what is the only amine hormone not derived from tyrosine?
melatonin
what is melatonin derived from?
tryptophan
are amine hormones hydrophilic or hydrophobic?
hydrophylic
are steroid hormones hydrophilic or hydrophobic?
hydrophobic / highly lipophilic
describe the mechanism of action of steroid hormones?
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.
what is the half life of steroid hormones versus amine hormones
Half life 60-90mins (vs 2mins for amine hormones)
what is the onset of action for steroid hormones?
Slow onset of action but action persists much longer than that of peptide or amine hormones.
what are steroid hormones produced by?
Gonads (testes and ovary) – sex steroids
Placenta - hCG (measures in pregnacy test), sex steroids
Kidney - Vitamin D3
Adrenal cortex - corticosteriods
describe the physiological activity of lipophilic hormones?
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]
summarise peptide and catecholamine hormones?
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.
summarise steroid and thyroid hormones?
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.
