The posterior pituitary

Question 1

Posterior pituitary

Answer 1

Direct anatomical connection with the hypothalamus, extension of the same brain tissue. Neurosecretory neurones frkm thehupothalamus synapse directly onto the systemic blood vessels in the posterior pituitary

Question 2

Posterior pituitary hormones

Answer 2

ADH And oxytocin are secreted from the posterior pituitary. They are Synthesised in the Supraoptic and Paraventricular nuclei of the hypothalamus and pass down into the posterior pituitary through modified axons. There are no stores of posterior pituitary hormones. Both are nanopeptides and very similar in structure.

Question 3

ADH (vasopressin) - synthesis and control

Answer 3

Synthesised hy Supraoptic nuclei in the hypothalamus.
Large peptide (9aa)
Transported attached to a carrier protein (neurophysin) to the posterior pituitary from where it is released.
Osmoreceptors in the hypothalamus measure the osmolarity of blood.
Decreased osmolarity give decreased ADH release
It is also controlled by blood volume receptors in the left atrium, increased blood volume gives decreased ADH secretion.

Question 4

Actions of ADH

Answer 4

ADH is the main controller of water homeostasis, its release is inhibited by alcohol (diuresis)
In the kidney it increases water permeability of the nephrons (water retention) by acting on the V2 receptor.
In blood vessels it causes Vasoconstriction by acting on V1 receptors.
Also has influence on sensitivity to pain and quality of sleep.

Question 5

ADH V2 receptor action

Answer 5

Acts on V2 receptors in distal convoluted tubules and collecting ducts of the kidney. This activates adenylate cyclase which increases cAMP. AQP-2’s are inserted into the luminal side of tubule cells (which are normally impermeable to water) water channels on the basolateral border (AQP-3/4) are always available.

Question 6

ADH V1 receptor action

Answer 6

Acts on V1 receptors on Vascular (arteriole) smooth muscle.
Minor physiological action but used in pharmacology.
Its action increases intracellular free Ca causing vasoconstriction, this increases BP.
Used to keep LA’s near site of action.

Question 7

Problems of ADH secretion

Answer 7

Diabetes insipidus

Carcinoma

Question 8

Diabetes insipidus

Answer 8

Due to a lack of secretion or action of ADH.
Can produce up to 20L urine per day
Lack of control of water homeostasis and polyuria.
Can be of pituitary or renal origin
Pituitary origin when there is lack of ADH for secretion
Renal when there is lack of action of ADH either due to a receptor or AQP problem.
Pituitary DI Treated with ADH analogue (Desmopressin)
Renal DI treated with thiazides. (paradoxical as these are diuretics but have opposite effect in renal DI) and indomethacin (May decrease glomerular filtration rate)

Question 9

Carcinoma

Answer 9

Causes too much ADH secretion, give anatagonist to treat

Question 10

Oxytocin

Answer 10

Synthesised in the Paraventricular nuclei in the hypothalamus.
Stimulates contraction of uterine muscle in childbirth (pharmacological use - to induce Labour and prevent post-partum bleeding)
Also stimulates milk let down during breast feeding, acts on myoepithelial cells in the mammary glands, nasal spray available to help those who have trouble breast-feeding.
Increased release in females during intercourse and in Males during ejaculation.