1b Disorders of Vasopressin Flashcards
What is the physiological action of vasopressin?
Stimulation of water reabsorption into the renal collecting duct = concentrates the urine
Which receptor does AVP work through?
V2 receptor in the kidney
How does AVP work as a vasoconstrictor?
Via the V1 receptor
Describe how AVP concentrates the urine?
- AVP binds to V2 receptor on basolateral membrane
- This triggers intracellular cascade resulting in the movement of aquaporin-2 molecules onto the apical membrane
- this allows the movement of water from the urine into the blood, therefore concentrating it
What is the bright spot on an MRI of the pituitary?
Posterior Pituitary
What are the two stimuli for vasopressin release?
Osmotic and Non-osmotic
Describe how the osmotic stimuli for vasopressin release works?
rise in plasma osmolality detected by osmoreceptors as they shrink when H2O released caused by high extracellular sodium.
there is increased osmoreceptor firing causing AVP release from hypothalamic neurons
Describe how the non-osmotic stimuli for vasopressin works?
decrease in atrial pressure sensed by atrial stretch receptors
Where do the neurones of the organosum vasculosum and subfornical organ project to?
the supraoptic nuclei
Describe how osmoreceptors regulate vasopressin?
- Increase in extracellular sodium
- Therefore movement of water out the osmoreceptor by diffusion
- Osmoreceptor shrinks
- This increases osmoreceptor firing
- More osmoreceptor firing leads to more AVP release from the hypothalamic neurones
How doe AVP trigger an increase in blood volume following haemorrhage?
Increased water absorption in the kidney via V2 receptor, therefore increasing blood volume and hence pressure as well
Which receptors does AVP initiate vasoconstriction through?
V1 receptors
What is the physiological response to water deprivation?
- Increased plasma concentration
- This stimulates the osmoreceptors to fire, leading to AVP release
- This stimulates water reabsorption from the renal collecting ducts
- Urine volume decreases, and the concentration of the urine increases as more water is kept
- This reduces the plasma concentration
What is the most common cause of polyuria, nocturia and polydipsia?
diabetes mellitus
What are the common symptoms in AVP deficiency and reisstance?
Polyuria
Polydipsia
Nocturia
Thirst
What is the difference between AVP deficiency and resistance?
Deficiency is due to a problem with the hypthalamus and posterior pituitary whereas resistance is a problem with kidney not being able to respond
What is the typical presentation of diabetes insipidus
Polyuria, nocturia, thirst and polydipsia
What happens to the urine of a patient with diabetes insipidus?
Very dilute urine (hypoosmolar) and large volumes
What happens to the plasma of a patient with AVP deficiency/resistance?
plasma concentration increases and becomes hyper
osmolar, as the patient becomes dehydrated as they are continually urinating large volumes of dilute urine
what happens to the sodium concentration in the plasma of a patient with diabetes insipidus?
increases
Why do patients with AVP deficiency/resistance have polydipsia?
The AVP problem results in impaired concentration of urine in the renal collecting duct
Therefore large volumes of dilute urine are produced
Increases in plasma osmolarity and sodium
Stimulation of osmoreceptors, leading to polydipsia
How can AVP deficiency/resistance lead to death?
When the patient has polydipsia but no access to water -this
leads to dehydration and death
What are causes of AVP defieciency?
Traumatic brain injury
Pituitary surgery or tumour
Metastasis to pituitary gland
Granulomatous infiltration of pituitary stalk
AUTOIMMUNE
Congenital is rare
What are the causes of AVP resistance?
Less common than deficiency
CONGENITAL- rare e.g. mutation in gene encoding V2 receptor
ACQUIRED
drugs e.g. lithium
