Disorders of Vasopressin Flashcards
What is the role of ADH?
Stimulate water reabsorption in the renal collecting duct
Distinguish between V1 & V2 receptors
V1 receptors have a role in vasoconstriction
V2 receptors are present on the basolateral membrane of the CD to concentrate the urine and promote water reabsorption
What are the 2 stimuli fro vasopressin release?
Osmotic - osmoreceptors detect rise in plasma osmolarity
Non-osmotic - atrial stretch receptors sense a decrease in atrial pressure
Describe the osmotic release of vasopressin
Organum vasculosum and subfornical organ sit around the 3rd ventricle; the axons project into the supraoptic nuclei, where vasopressinergic neurones are found; there is no blood brain barrier so neurones can respond to changes in systemic circulation.
Explain the functioning of osmoreceptors
Osmosreceptors detect a rise in extracellular Na+, thus water leaves the osmoreceptors causing shrinking and AVP is released stimulating water reabsorption
How do atrial stretch receptors work?
They detect pressure changes in the right atrium. When circulating blood volume decreases e.g. in haemorrhage, there is less stretch of the receptors so less inhibition of AVP and so it’s released
What are the main symptoms of diabetes inspidus?
Polydispia
Nocturia
Polyuria
Thirst
Distinguish between the 2 types of diabetes insipidus
Cranial/Central/Vasopressin Insufficiency - where there is an absence of AVP due to issues with the hypothalamus or posterior pituitary gland
Nephrogenic - when there is resistance to AVP, it’s produced but V2 receptors are not responsive
Causes of cranial DI
Metastasis from another organ i.e. breast
Pituitary tumour
Pituitary surgery
Autoimmune
Brain trauma
Granulomatous infiltration of infundibulum
Congential - rarer
Causes of Nephrogenic DI
Congenital - mutations in V2 receptor gene or aquaporin 2 channel
Acquired - drugs like lithium
What are some presentations of DI?
Hypernatraemia
Very dilute urine
Hyper-osmolar blood thus dehydrated
Glucose is normal
What is psychogenic polydipsia
Similar presentations to DI but there are no issues with AVP
The issue is that the patient drinks excessive water
It’s psychological
How to distinguish between psychogenic polydipsia and DI?
Water deprivation test - measure urine volume and conc. of urine and plasma
Weigh regularly and if body weight drops by more than 3% it indicates DI
In DI the patient is also unable to concentrate their urine
What is ddAVP?
Desmopressin - synthetic AVP
How to distinguish between cranial and nephrogenic DI?
Following administration of desmopressin:
Cranial - urine concentrates
Nephrogenic - urine doesn’t concentrate as kidneys cannot respond
