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Adrenal Hormones
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Adrenal hormonesStructure and function (lecture 1)
The adrenal glands release hormones.
There are 4 main types:
Mineralocorticoids- such as aldosterone: this regulates Na and K.
Glucocorticoids- such as cortisol: this regulates glucose metabolism.
Sex steroids- such as androgens: these supplement the sex steroids from the testes
and ovaries.
Catecholamines- such as adrenaline, noradrenaline, dopamine: “fight or flight” and
arousal, attention, memory and learning, reward attention and movement control.
The adrenal gland:
Adrenal glands sit on top of each of our kidneys.
They are highly vasculated so have lots of blood flow.
Adrenal gland has various layers: it has a capsule around the outside, then the cortex
and then the medulla is in the middle.
The main vasculature is through the medulla in the middle, where all the signals are
received. Because it is a hormone so some of the signals are received but also some
of the hormones that they produce are released into the blood system so that they
can be circulated around the body.
Structure of the adrenal gland:
Cortex is on the outside and the medulla is in the middle.
Specifically, the medulla produces the catecholamines.
The cortex is made up of 3 different layers:
Zona glomerulosa is where the mineralocorticoids are made.
Zona fasciculata is where the glucocorticoids are made.
Zona reticularis is where the androgens are made.
Catecholamine biosynthesis:
The main substrate or precursor is the amino acid tyrosine.
Through a series of steps dopamine is made
Dopamine is either made and secreted or undergoes further biosynthesis and makes
noradrenaline. Noradrenaline can be secreted or it can be converted into adrenaline.
Biosynthesis of steroid hormones:
2 of the steroid hormones: cortisol and aldosterone
The precursors of these hormones are cholesterol.
Cholesterol is converted through pregnenolone, through progesterone and then
through various other enzymes converting it to either cortisol or aldosterone.
Progesterone can also be converted into testosterone and estrodial.
So cholesterol is an important precursor for many of these steroid hormones.
Differential stimuli between cortex & medulla 1.
, We have various differential stimuli between the cortex and medulla of the adrenal
glands.
The anterior pituitary gland produces ACTH upon stimulation from the corticotrophin
releasing hormone CRH from the hypothalamus.
So the hypothalamus produces CRH, stimulates the anterior pituitary and that
produces ACTH which is then released into the blood stream and that is then
targeted to the adrenal cortex, the outside layer of the adrenal glands.
The adrenal medulla is innovated by nerves, the preganglionic sympathetic nervous
system, via the hypothalamus, with innovations straight through the spinal cord,
through the sympathetic fibres to the adrenal medulla, where there are very specific
cells within the adrenal medulla, which produce catecholamines which are then
released into the blood stream and can get circulated around the body.
The responses to stress are usually short term:
Increased heart rate.
Increased blood glucose.
Increased blood pressure.
Long term stress response:
Increased protein breakdown.
Increased lipolysis.
Increased gluconeogenesis.
Feedback mechanisms
With a hormone there is a system, that system may produce a hormone which has a
direct effect on another tissue.
Sometimes this can be a positive effect but sometimes this can be a negative effect,
with respect to the feedback loops.
Neuroendocrine axis
The brain, the hypothalamus which signals to the pituitary (anterior for example).
The released hormones from the anterior pituitary would have an effect on another
gland.
There may be some feedback mechanisms back up to the brain, the hypothalamus
and into the anterior pituitary.
Releasing hormones:
The releasing hormones, secreted via the hypothalamus to the anterior pituitary.
The anterior pituitary then releases trophic hormones.
The trophic hormones would have an action at the target gland.
The hormone would then go and have an effect on its various tissue types.
But would also have negative feedback on the anterior pituitary and also the
hypothalamus.
Examples:
CRH-> ACTH
LHRH->LH
TRH->TSH
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