Posterior Pituitary Gland and the Role of Oxytocin and Vasopressin

Question 1

Anterior Pituitary secretes __ principle hormones

Answer 1

6

Question 2

Posterior pituitary secretes __ principle neurohormones

Answer 2

2

Oxytocin and Vasopressin

Question 3

Oxytocin and Vasopressin are produced by __________________

Answer 3

Oxytocin and Vasopressin are produced by magnocellular hypothalamic neurones.
-> they have large cell bodies, in the supraoptic and paraventricular nuclei of the hypothalamus

Question 4

Magnocellular neurone key steps:

Answer 4

1. uptake of precursors
2. Synthesis of pro hormone
3. Packaging int granules
4. Fast axonal transport (2mm/h)
5. Cleavage of pro hormone
6. Storage

Question 5

Features of a magnocellular neurone

Answer 5

• Rough ER
• Golgi complex
• Pituitary stalk

Question 6

Recall the Stages in stimulus secretion coupling within an anon terminal of the post. pituitary

Answer 6

1. Arrival of Action potential
2. Depolarisation by action potential; influx of sodium ions
3. Opening of Calcium Chanels
4. Exocytosis of granule contents (oxytocin and Neurophysin)
5. Recapture of granule membrane
6. Calcium incorporation into microveiscles and/or extrusion
7. Restoration of membrane potential by a NA+-K+ pump

Question 7

Oxytocin

Answer 7

• Nanopeptide 9 amino acids
• causes contraction of the uterine wall
• used to induce uterine contractions in parturition
• involved in milk ejection
• causes contraction of smooth muscle cell types surrounding milk duct of mammary gland
• short circulatory half life ~5 min
• circulates as free peptide mostly cleared by the kidney

Question 8

Vasopressin

Answer 8

• Nanopeptide, similar in structure to oxytocin
• Reduces water secretion by kidney = treats diabetes insipidus
• potent vasoconstrictor = treat bleeding and blood pressure
• short circulatory half life ~5mins
• circulates as free peptide, mostly cleared by kidney

Question 9

Vasopressin and Oxytocin act via G protein coupled receptors

Answer 9

• two subtypes of vasopressin receptor, V1 and V2

Question 10

Activation of Oxytocin and type V1 vasopressin receptors…..

Answer 10

receptor promotes smooth muscle contraction by activation of PHOSPHATE C pathway:
increasing PI turnover -> mobilising intracellular calcium

Question 11

Activation of Vasopressin receptor V2

Answer 11

this increases water permeability of renal tubes.

Mediated by the action of ADENYLATE CYCLASE and cAMP- DEPENDANT PROTEIN KINASES

Question 12

Vasopressin secretion control:

Answer 12

main physiological stimulus: increased osmotic pressure of blood - monitored by osmoreceptors in the hypothalamus.
Also regulated by:
- fall in blood volume (>8%)
- reduced arterial pO2
- raised temp
- pain, trauma, infection

Question 13

VASOPRESSIN (ADH): at normal concentrations its actions …..

Answer 13

on the renal nephron:
increase permeability of renal distal convoluted tube and collecting ducts of water:
=more reabsorbed from tubules
=reduced urine output; more concentrated
=restoration of blood osmolarity and volume

Question 14

At HIGHER concentrations ADH acts on blood vessels and causes…

Answer 14

Constriction of blood vessels

raise blood pressure

Question 15

What are the consequences of excessive vasopressin secretion?

Answer 15

Fluid retention and Decreased plasma osmolarity

Question 16

What is the treatment for excessive Vasopressin secretion?

Answer 16

> limit fluid intake (500ml/day)

>tumour removal?

Question 17

What can cause Insufficient Vasopressin secretion?

Answer 17

occurs after head injury/ damage to the hypothalamic-posterior-pituitary/ rare mutation preventing vasopressin production.

Question 18

Consequences of insufficient vasopressin secretion

Answer 18

• Diabetes Insipidus
• Copious, dilute, urine
• Intense thirst
• Polydipsia (thirst)

Question 19

Treatment for insufficient vasopressin secretion?

Answer 19

nasal spray or injection of a synthetic analogue of vasopressin

Question 20

Oxytocin

Answer 20

Suckling induces release of oxytocin-> neuroendocrine reflex

Question 21

Neuroendocrine reflex controlling milk ejection

Answer 21

tactile stimulation at nipple
sensory nerves - spinal cord - brain stem - oxytocin producing MAGNOCELLULAR NEURONE
- transport via blood to MYO-EPITHELIAL Cells (ME) lining alveoli of mammary gland
-contraction of ME cells raises a pressure in alveoli squeezing milk into ducts and cistern
-sudden rise in intermammary pressure can cause milk to squirt from nipple/teat

Question 22

What kind of reflex is the release of oxytocin

Answer 22

Condition reflex- top baby cry. STRESS can inhibit milk ejection reflex.

Question 23

Milk Ejection involves a neuroendocrine _________________ loop

Answer 23

Positive feedback

Question 24

Describe the positive feedback loop of milk ejection

Answer 24

hypothalamic-hypohyseal tract. increasing firing rate after nipple or reproductive tract stimulation causes increased oxytocin secretion. increase oxytocin release, increased oxytocin conc in myoepithelial cells

Question 25

What else cause oxytocin secretion?

Answer 25

Tactile stimulation of female reproductive tract i.e. vagina, cervix.
especially in late pregnancy / post part period
contraction of uterus = expulsion of foetus