Neurohypophysial Disorders

Question 1

Name the two main nuclei within which neurones of the neurohypophysis have their cell bodies.

Answer 1

Paraventricular Nucleus

Supraoptic Nucleus

Question 2

What two hormones are produced by the neurohypophysis?

Answer 2

Vasopressin

Oxytocin

Question 3

What is the principal action of vasopressin and how does it carry out this action?

Answer 3

Vasopressin’s main action is on the V2 receptors in the renal cortical and medullary collecting ducts

It stimulates the synthesis and assembly of aquaporin 2, which then increases water reabsorption and has an antidiuretic effect

Question 4

State some other actions of vasopressin.

Answer 4

Vasoconstriction

Corticotrophin release

Factor VIII and von Willebrand factor

Central effects

Question 5

What are the main actions of oxytocin?

Answer 5

It is a contractile molecule that binds to oxytocin receptors

It causes contraction of the myometrium during parturition and is involved in milk ejection

It also has central effects

Question 6

What are the consequences of a lack of the neurohypophysial hormones?

Answer 6

Lack of Oxytocin – not clinically significant

Lack of Vasopressin – Diabetes Insipidus

Question 7

What are the two forms of diabetes insipidus?

Answer 7

Central (cranial) and Nephrogenic Diabetes Insipidus

Question 8

What can cause central diabetes insipidus?

Answer 8

Damage to neurohypophysial system (injury, surgery, cerebral thrombosis, tumours, granulomatous infiltration)

Idiopathic

Familial (rare)

Question 9

What can cause nephrogenic diabetes insipidus?

Answer 9

Familial (rare)

Drugs e.g. lithium, dimethyl chlortetracycline (DMCT)

Question 10

State some signs and symptoms of diabetes insipidus.

Answer 10

Polyuria

Polydipsia

Hypo-osmolar urine

Dehydration

Possible disruption of sleep

Possible electrolyte imbalance

Question 11

State another cause of polydipsia that isn’t diabetes.

Answer 11

Psychogenic polydipsia

This is a central disturbance that increases the drive to drink

Question 12

What test can be used to distinguish between normal, psychogenic polydipsia, central DI and nephrogenic DI? Describe the results you would expect.

Answer 12

Fluid deprivation test
 Normals and psychogenic polydipsia will show a rise in urine osmolality
 Central and nephrogenic diabetes insipidus will show little or no change in urine osmolality

Fluid deprivation with administration of DDAVP (Desmopressin)
 Central diabetes insipidus will show a rise in urine osmolality
 Nephrogenic DI will still have a low urine osmolality (because of end-organ resistance)

Question 13

Why is the urine osmolality of someone with psychogenic polydipsia lower (in the fluid deprivation test) than a normal subject?

Answer 13

Over time, the constant passage of large volumes of water through the kidneys will wash out the osmotic gradient that is necessary for AVP to exert its diuretic effect

Question 14

Describe the normal change in urine osmolality as plasma osmolality increases.

Answer 14

Normally, urine osmolality will increase as plasma osmolality increases (in a graph of urine osmolality against plasma osmolality it will show a sigmoid shape)
In DI, there is little change in urine osmolality as plasma osmolality increases

Question 15

Describe changes in plasma vasopressin following administration of hypertonic saline in a normal subject, psychogenic polydipsia, central DI and nephrogenic DI.

Answer 15

Hypertonic saline will increase the plasma osmolality and hence will increase the vasopressin secretion in patients that have the capacity to produce vasopressin (normal, psychogenic polydipsia and nephrogenic DI)

Patients with central DI can’t produce vasopressin at all so the hypertonic saline will show no change in plasma vasopressin

Question 16

What is SIADH?

Answer 16

Syndrome of Inappropriate ADH = when the plasma vasopressin concentration is inappropriate for the existing plasma osmolality

Question 17

State some signs of SIADH.

Answer 17

Decreased urine volume Increased urine osmolality

Question 18

What is the main consequence of SIADH?

Answer 18

HYPONATRAEMIA

Question 19

State some symptoms of SIADH that are caused by the hyponatraemia.

Answer 19

At relatively mild hyponatraemia = generalized weakness, poor mental function, nausea

Severe hyponatraemia = confusion, coma, death

Question 20

State some causes of SIADH.

Answer 20

Tumours (ectopic secretion)

Neurohypophysial malfunction (e.g. meningitis, cerebrovascular disease)

Thoracic disease (e.g. pneumonia)

Endocrine disease (e.g. Addison’s)

Physiological – it can happen under normal circumstances where AVP is release is stimulated by non-osmotic stimuli (e.g. hypovolaemia, pain, surgery)

Drugs

Idiopathic

Question 21

How is SIADH treated?

Answer 21

Fluid Restriction

Provide appropriate treatment when the cause is identified (e.g. surgery for a tumour)

Long term- can give drugs to induce nephrogenic DI (eg: demeclocyline)

Vaptans: non-competitive V2 antagonist

NOTE: if someone is hyponatraemic you need to deal with that as soon as possible – e.g. use drugs that prevent vasopressin action in the kidneys

Question 22

What is the name given to exogenous vasopressin?

Answer 22

Argipressin

Question 23

Where are V1 and V2 receptors found?

Answer 23

V1
 Vascular smooth muscle 
 Non-vascular smooth muscle 
 Anterior pituitary
 Liver 
 Platelets 
CNS

V2
 Kidney
 Endothelial cells

Question 24

State the pharmacological actions of argipressin.

Answer 24

NATRIURESIS – this is one of the unexplained side-effects of having a large amount of vasopressin – it is V2 mediated and only happens when given at high doses

PRESSOR ACTION – V1 mediated vasoconstriction – the coronary vessels are particularly sensitive to vasopressin (this can cause cardiac ischaemia and angina attacks)
Contraction of vascular smooth muscle

Increased ACTH secretion

Increased factor VIII and vWF production

Question 25

State two selective peptidergic vasopressin selective agonists.

Answer 25

V1 – Terlipressin

V2 - Desmopressin

Question 26

Compare the effects of argipressin and desmopressin.

Answer 26

Argipressin acts on V1 and V2

Argipressin is more effective in causing vasoconstriction via V1 receptors

Desmopressin is more effective in the kidneys in causing water reabsorption via V2 receptors

Question 27

State some clinical uses of Desmopressin.

Answer 27

Treatment of diabetes insipidus

Treatment of nocturnal eneuresis

Haemophilia (need V2 stimulation)

NOTE: it is taken orally or nasally

Question 28

State some unwanted effects of Desmopressin.

Answer 28

Nausea

Headaches

Abdominal pain

Fluid retention and hyponatraemia

Question 29

State two peptidergic V1 receptor agonists and their uses.

Answer 29

Terlipressin – used to treat oesophageal varices (it causes vasoconstriction)

Felypressin – injected by dentists along with local anaesthetic (the vasoconstriction keeps the local anaesthetic at the site of injection for longer thus prolonging the action of the local anaesthetic)

Question 30

State one treatment used for nephrogenic diabetes insipidus and its possible mechanism of action.

Answer 30

Thiazide Diuretics (e.g. bendroflumethiazide)

This inhibits the Na+/Cl- pump in the DCT leading to a diuretic effect

This leads to volume depletion resulting in a compensatory increase inNa+ reabsorption from the PCT

This increases proximal water reabsorption so less water reaches the collecting duct

This ultimately leads to reduced urine volume

Question 31

What are vaptans?

Answer 31

Non-peptide vasopressin receptor antagonists

Tolvaptan = V2 receptor antagonist

It is used to treat hyponatraemia associated with SIADH and may be useful in treating congestive heart failure

Question 32

State some drugs that increase or decrease vasopressin secretion.

Answer 32

Increase vasopressin secretion = nicotine

Decrease vasopressin secretion = alcohol + glucocorticoids