DISORDERS OF VASOPRESSIN Flashcards
What are the 3 effects of AVP and their respective receptors?
Water reabsorption in renal collecting duct by V2 receptor
Vasoconstrictor via V1 receptor
Stimulates ACTH release from anterior pituitary
What are the 2 stimuli for AVP release?
- Rise in plasma osmolality sensed by osmoreceptors
- Decrease in atrial pressure sensed by atrial stretch receptors
What are the names of the two nuclei for the osmotic stimulation of vasopressin release?
Organum vasculosum
Subfornical organ
Describe how the organum vasculosum and subfornical organ carry out their function.
They sit around the 3rd ventricle where there is no blood barrier so they can respond to systemic changes. Neurones project down to the supraoptic nucleus.
How do osmoreceptors work?
When plama osmolality increases, water diffuses out of osmoreceptor cell and it shrinks causing increase osmoreceptor firing
Where do atrial stretch receptors detect pressure?
Right atrium
How do atrial stretch receptors inhibit AVP release?
Inhibit AVP release via vagal afferents to hypothalamus
e.g. haemorrhage = less circulating volume = less inhibition of AVP
What are the 2 effects of osmoreceptor stimulation?
Thirst and AVP release
What is the difference and similarity between diabetes insipidus and mellitus?
similarities: polyuria, nocturia, thirst, polydispia
differences: in mellitus symptoms due to hyperglycaemia and osmotic diuresis, in insipidus symptoms due to problem with AVP
What are the 2 types of diabetes insipidus?
Cranial and Nephrogenic
What is the difference between CDI and NDI?
CDI - problem with hypothalamus/pituitary causing inability to make AVP
NDI - problem with kidney (collecting duct) where its unable to respond to AVP
List causes of CDI
Brain trauma, pituitary surgery, adenoma or metastasis from another site, granulomatous infiltration of pituitary stalk (TB, sarcoidosis…), autoimmune, congenital
List causes of nephrogenic diabetes insipidus
Drugs e.g. lithium (used in mental health)
Congenital e.g. mutation in gene encoding V2 receptor/aquaporin 2
Clinical presentations of DI?
Dilute, large volume of urine
Increased plasma conc.
Hypernatraemia (increased sodium conc in plasma)
Normal blood glucose level
What condition often mimics diabetes insipidus?
Psychogenic polydipsia
What is psychogenic polydipsia?
Patient has a psychological problem involving the consumption of vast amounts of water which can present the same symptoms as DI
Name the test used to distinguish between DI and PP and explain it
Water deprivation test - Don’t let patient drink and fluid and overtime measure urine volume and concentration of urine and plasma. Weigh the patient regularly and if > 3% loss of weight stop test (significant dehydration which occurs in DI)
How do we distinguish between CDI and NDI?
Give ddAVP (synthetic AVP - V2 receptor agonist) during water deprivation test.
If CDI urine osmolality should increase and volume decrease
If NDI nothing will happen
What is the difference in plasma osmolality between DI and psychogenic polydipsia?
DI - high osmolality (290)
PP - low osmolality (270)
What is the treatment for cranial DI?
Replace AVP with desmopressin (oral/intranasal)
What is the treatment for nephrogenic DI?
Give thiazide diuretics e.g. benzofluazide (mechanism is unclear)
Very difficult to treat successfully
What is SIADH?
Syndrome of Inappropriate Anti-Diuretic Hormone
- too much AVP causing low, conc. urine and water retention
What are some causes of SIADH?
Head injury, stroke, tumour
Pulmonary disease (pneumonia, bronchiectasis)
Lung cancer
Drug related e.g. carbamazepine, serotonin reuptake inhibitors (SSSRIs)
Idiopathic
Describe the management of SIADH
Prolonged hospital stay until sodium conc. in plasma increases Fluid restrict Vasopressin antagonist (vaptan) - binds to V2 receptor however extremely expensive
