1b. Overview continued + Steroid & Amine hormones Flashcards
What are steroid hormones derived from
cholesterol
When are steroid hormones synthesised and why is this
synthesised directly as needed as they are highly lipophilic (lipid soluble) so cannot be retained within lipid membranes
What happens to steroid hormones once they are synthesised
they diffuse across the membrane into the ISF and the blood
how are steroid hormones transported through the body
as they are poorly water soluble they are transported bound to carrier proteins e.g. ALBUMIN
Why are steroid hormones bound to carrier proteins
to stabilise transport throughout the body and protect them from enzymatic degradation (phenomenally increases their half life to around 60-90 mins instead of 2mins)
where in the body are steroid hormones produced and give examples at each location
- gonads (testes/ovaries) - sex steroids
- placenta - hCG, sex steroids
- Kidneys - Vitamin D3
- Adrenal cortex - corticosteroids
what determines which specific steroid hormone is ultimately produced from cholesterol
different cells having different enzymes synthesising different derivatives of cholesterol - this common ancestry explains why there are cross effects of excess steroids (e.g. see body builders)
where are steroid hormone receptors located on the cell and why
receptors are located inside the cell (cytoplasmic or nuclear receptors) because they are lipophilic so can cross the plasma membrane easily
what do steroid hormones trigger when they bind to receptors
either activation or inhibition of gene function within the nucleus = GENOMIC EFFECT
genes control the synthesis of protein so these hormones either increase or decrease protein synthesis
why is there a lag time between steroid hormone release and biological affect (hours to days)
because protein synthesis is a relatively slow process - the effects however will persist for around the same time though
how would a steroid hormone produce a rapid response
by occasionally binding to cell surface receptors and using 2nd messenger systems
What are amine hormones derived from
one of two amino acids
- Tyrosine
- Tryptophan
What is the only amine hormone derived from tryptophan and what is its role
melatonin - regulates circadian rhythm
amine hormones derived rom tyrosine can be split into what two groups
- catecholamines
2. thyroid hormones
name three catecholamines and where they are secreted from
- dopamine - brain
- norepinephrine - neurons
- epinephrine - adrenal medulla
what is the mechanism of action for catecholamines
similar mechanism to peptide hormones (hydrophilic)
name 2 thyroid hormones
- Triiodothyronine (T3)
2. Thyroxine (T4)
What is the mechanism of action for thyroid hormones
similar mechanism to steroid hormones (lipophilic)
what are “free hormones”
the small amount of lipophilic (steroid/thyroid) hormones that are unbound within the plasma (i.e. should have a carrier but do not)
what is the free hormone: hormone-protein complex ratio and what is it in favour of
the ratio between the number of free hormones to complexed (protein bound) hormones in the plasma
the ratio is much in favour of bound hormones
what can free hormones do that bound hormones can’t
diffuse across capillary walls to target cells
what does the law of mass action dictate in terms of free hormones
as free hormones leave the plasma and are taken up by cells, more hormones are released from their carriers to maintain the ratio - allows for a reservoir of hormones to be ready for action
What does hormone concentration in the blood depend on
rate of secretion and rate of removal
how does removal of hormones occur
excretion or metabolic transformation - mainly in the liver and kidneys
why are catecholamine and peptide hormones excreted quickly/easily
they have a short half life in plasma (mins to hours) as they are unbound so are metabolised or excreted very quickly
why does it take longer for steroid and thyroid hormones to be excreted
they have a long half life (hours to days) as they are protein bound so take much longer to be metabolised or excreted
what are the two main main control mechanisms endocrine pathways will respond to
- negative feedback reflexes - e.g. parathyroid hormone
2. neural feedback loops e.g. adrenaline
How do neural feedback loops work using adrenaline as an example
- fright/flight/fight response elicited
- generalised sympathetic discharge including activation of sympathetic fibres to adrenal medulla occurs
- specialised cells in adrenal medulla release adrenaline into the blood stream
give an example of a hormone whose secretion can be subject to multiple control mechanisms
insulin - plasma glucose concentration, autonomic nerve activity, presence of food in gut, presence of additional hormones in the plasma
What is up-regulation and when does it occur
the increase in the number of hormone receptors on target tissues to increase the tissue sensitivity to the hormone -occurs after prolonged exposure to low hormone concentration in the plasma
what is down regulation and when does it occur
the decrease in the number of hormone receptors on target tissues to decrease the tissue sensitivity to the hormone - occurs after prolonged exposure to high hormone concentration in the plasma
what is a permissive effect
when the presence of one hormone enhances the effect of another e.g. the presence of thyroid hormone (TH) increases synthesis of receptors for epinephrine on adipocytes greatly increasing epinephrines ability to release fatty acids
what is the clinical relevance to be aware of for hormone levels in the body
may be misleading as hormone release occurs in short bursts so concentration in plasma be may not prove accurate - 24hr monitoring is required to give a true depiction of hormone levels
