Disorders of Vasopressin Flashcards
What part of the pituitary is continuous with the hypothalamus?
Posterior pituitary
What are the 2 key hormones of the posterior pituitary?
AVP
Oxytocin
What neurones contain AVP and oxytocin?
Hypothalamic magnocellular neurones
What is the other name for vasopressin?
Anti diuretic hormone
What is the main physiological action of vasopressin?
Stimulates reabsorption of water in renal collecting duct, concentrating the urine
What are the other 2 minor roles of vasopressin?
Vasoconstrictor via V1 receptor
Stimulates ACTH release
How does vasopressin concentrate urine?
Binds to V2 receptor in collecting duct
Intracellular signalling cascade
Aquaporin 2 channels travel to apical membrane
Water in via aquaporin 2, through cell, through aquaporin 3 on basolateral membrane back into blood
What is the best way to view the pituitary gland?
Pituitary MRI
On an MRI of the pituitary what is the bright spot?
On the posterior pituitary BUT not everyone has it
What are the 2 stimuli for vp release?
Osmotic- rise in plasma osmolality (plasma conc) sensed by osmoreceptors
Non-osmotic- decrease in atrial pressure sensed by atrial pressure receptors
What 2 nuclei are around the 3rd ventricular? What type of receptor do they contain?
Organum vasculosum and subfornical organ, they contain osmoreceptors
What is special about the organum vasculosun and subfornical organ
No blood brain barrier so respond to changes systemic circulation, neurones from here project to the supraoptic nucleus - site of vasopressin release
How is release of vp via osmoreceptors triggered?
Increase in extracellular Na+ Water moves out via osmosis Osmoreceptor shrinks Increased firing AVP release from hypothalamic neurones
Where do atrial strecth receptors detect pressure?
Right atrium
How do atrial stretch receptors work for vp release/inhibition?
When they are stretched and BP is normal/high, they inhibit release via vagal afferents to hypothalamus
If less stretched, less inhibition, more vp release
Why is vp released following heamorrhage?
Increases blood vol (V2 receptors) and vasoconstricts (V1 receptors)
What happens during water deprivation?
Plasma osmolality increases Osmoreceptors shrink and fire AVP release triggered, thirst stimulated Increased water reabsorbtion from collecting ducts Small vol of concentrated urine Reduction in plasma osmolality
What are physiological effects of diabetes insipidus?
Polyuria
Nocturia
Thirst (extreme)
Polydipsia
How is diabetes insipidus different to diabetes mellitus?
Mellitus= hyperglycaemia Insipidus= problem with AVP
What is the most common cause of polyuria/n octuria etc?
Diabetes mellitus
What are the 2 types of diabetes insipidus and their causes?
Cranial- unable to make AVP (acquired causes more common eg surgery, tumors, injury, autoimmune)
Nephrogenic- make AVP but collecting duct doesnt respond (much less common, most common cause is lithium, sometimes congenital)
How does lithium cause diabetes insipidus?
Nephrogenic- damages collecting ducts
How does diabetes insipidus present?
Very dilute urine Large vols of urine Increased plasma conc (hyperosmolar) Increased sodium (hyponatraemia) Normal glucose
How do you different between mellitus and insipidus?
Insipidus will have normal glucose, mellitus will have high glucose
How do we describe dilute urine?
Hypotonic
How is dehydration automatically prevented in those with diabetes insipidus?
Stimulation of osmoreceptors stimulates thirst so they drink more water
What condition mimics diabetes insipidus, what is it and how is it different?
Psychogenic polydipsia, theres no problem with AVP, they just drink a lot of water (osmolality falls, less AVP produced, lots of dilute water)
How do we differentiate between psychogenic polydipsia and diabetes insipidus?
Water deprivation test: over time measure urine vol, urine conc and plasma conc
DI= urine conc will still be very low and high vols of urine
Psycogenic polydipsia= urine conc will rise and vol of urine will fall
How do we differentiate between cranial and nephrogenic diabetes insipidus?
Give an injection of synthetic VP (called ddAVP)
Nephrogenic will not respond and will keep urinating large vols (problem in kidneys), cranial will respond as they just need AVP (problem with production)
What is the difference in plasma osmolality between diabetes insipidus and psycogenic polydipsia?
DI= osmolality is high PP= osmolality is low
What medication is given for diabetes insipidus?
Desmopressin
How do people die of diabetes insipidus in hospital?
Denied water as its often confused with diabetes mellitus- if not giving water you have to give them desmopressin
How is nephrogenic DI treated?
Thiazide diuretic (paradoxical and v confusing, mechanism unclear)
What is syndrome of inappropriate anti-diuretic hormone and how is it caused?
Too much AVP therefore reduced urine output and water retention
High urine osmolality, low plasma osmolality, dilutional hyponatraemia
CNS causes are common (injury, stroke, tumor), pulmonary disease (pneumonia), malignancy, drug related, idiopathic (lots of elderly people- we dont know why)
How is SIADH managed?
Fluid restrict
Use a VP antagonist (vaptan- binds to V2 receptors in kidney and blocks it but very expensive)
