(endo) disorders of vasopressin Flashcards
what type of hypothalamic neurones release AVP and oxytocin?
magnocellular hypothalamic neurones
what type of hypothalamic neurones release TSH/LH/FSH/prolactin/GH/ACTH?
parvocellular hypothalamic neurones
what are magnocellular neurones?
hypothalamic neurones that release AVP and oxytocin into the posterior pituitary
what are parvocellular neurones?
hypothalamic neurones that release LH/FHS/TSH/ACTH/GH/prolactin into the anterior pituitary
which hypothalamic nuclei do the magnocellular neurones originate in?
paraventricular AND supraoptic hypothalamic nuclei
what are the two types of nuclei in the hypothalamus?
paraventricular and supraoptic nucleus
which hypothalamic nuclei do the parvocellular neurones originate in?
paraventricular hypothalamic nucleus
what is AVP alternatively called?
anti-diuretic hormone (ADH)
define diuresis
production of urine
describe the main physiological action of vasopressin
stimulates water reabsorption in the renal collecting duct = concentrates urine
(maximising water reabsorption back into the bloodstream)
through what does vasopressin act to stimulate water reabsoprtion?
via the V2 receptor
what is the physiological effect of vasopressin acting on the V1 receptor?
vasoconstriction
what is the physiological effect of vasopressin acting on the V2 receptor?
stimulates water reabsoprtion in the renal collecting duct to concentrate urine
what is the physiological effect of vasopressin acting on the V3 receptor?
stimulates the release of ACTH from the anterior pituitary gland
what impact does vasopressin release have on the pituitary gland?
stimulates ACTH release from the APG, via the V3 receptor
explain, in detail, how vasopressin acts to concentrate urine
AVP in the bloodstream binds to V2 receptors on the surface of renal collecting duct cells
binding stimulates an intracellular signalling cascade
cascade causes the insertion of aquaporin-2 channels into the apical membrane of the collecting duct cells
water molecules enter the CD cells via aquaporin-2 channels from the tubular lumen
molecules travel down a concentration gradient and enter the bloodstream via aquaporin-3 channels on the basolateral membrane
how is the posterior pituitary visualised usually on an MRI?
posterior pituitary ‘bright spot’
is the posterior pituitary ‘bright spot’ present in all MRIs?
no, absense of the bright spot is normal - not visualised in all healthy individuals
what are the stimuli for vasopressin release?
osmotic stimuli and non-osmotic stimuli
describe the osmotic stimulus for vasopressin release
increase in plasma osmolality (concentration) detected by osmoreceptors
describe the non-osmotic stimulus for vasopressin release
decrease in atrial pressure sensed by atrial stretch receptors
what is plasma osmolality?
measure of the plasma electrolyte conenctration
when does plasma osmolality increase?
dehydration
what detects plasma osmolality?
osmoreceptors in the organum vasculosum and the subfornical organ (nuclei found around the third ventricle)
what are the subfornical organ and the organum vasculosum?
nuclei around the third ventricle that contain osmoreceptors that detect plasma osmolality
collection of neuronal cell bodies
where are the subfornical organ and the organum vasculosum located?
around the third ventricle
what feature of the subfornical organ and the organum vasculosum enables neuronal response?
no blood-brain barrier so the neurones can respond to changes in the systemic circulation
(+ highly vascularised)
where do the neurones from the subfornical organ and the organum vasculosum project to?
supraoptic nucleus in the hypothalamus where vasopressinergic neurones lie
explain, in detail, the process by which osmoreceptors control vasopressin release
plasma sodium osmolality increases
stimulates water to move out of the osmoreceptor into the surrounding plasma causing the osmoreceptor to shrink
osmoreceptor shrinkage causes increased osmoreceptor firing
increased firing rate causes AVP release from the PPG
what stimulates osmoreceptor action?
increase plasma sodium osmolality
what impact does osmoreceptor shrinkage have on the firing rate?
increases osmoreceptor firing rate
what is the function of atrial stretch receptors?
detect atrial pressure in the right atrium
explain, in detial, the non-osmotic stimulation of vasopressin release
reduction in circulating volume = reduction in pressure
reduction in pressure detected by atrial stretch receptors that stretch less
reduced stretching = less inhibition of vasopressin release (i.e. more AVP released)
more vasopressin released = more vasoconstriction AND water reabsorption so pressure can normalise
(i.e less stretching = more vasopressin)
explain, in detail, the osmotic stimulation of vasopressin release
osmoreceptors shrink due to the stimulus of increased plasma sodium osmolality, increasing firing rate causing increased AVP release from the PPG
what impact does stretching have on vasopressin release?
less stretching = more vasopressin = more vasoconstriction AND water reabsorption
through what is vasopressin release inhibited?
via vagal afferents to hypothalamus
give two reasons why vasopressin is released following a haemorrhage
haemorrhage = reduction in circulating volume
1 – more vasoconstriction via V1 receptors to increase vascular pressure and prevent blood loss (!!)
2 – to stimulate more water reabsorption via the renal collecting duct via V2 receptors to restore some of the circulating volume
(less volume = less pressure = less stretch of atrial stretch receptors = more vasopressin)
why is vasopressin release important during a haemorrhage?
so vasoconstriction can take place to prevent further blood loss and to increase pressure in the circulatory system again
++
so concentrated urine can be produced and as much water as possible can be retained within the systemic circulation (to maintain circulating volume)
what is the physiological response to water deprivation?
plasma osmolality increases
stimulate osmoreceptors to shrink
osmoreceptor firing rate increases and stimulates vasopressin release from the PPG
vasopressin increases water reabsorption from the renal collecting duct via the V2 receptors
urine volume reduces while the urine osmolality increases
plasma osmolality reduces and eventually returns to normal
what symptoms can be seen in a patient with diabetes insipidus?
polyuria
nocturia
polydipsia (extreme thirst)
how common is diabetes insipidus?
extremely rare
what is the most common cause of polyuria, polydipsia and nocturia?
diabetes MELLITUS, not diabetes insipidus