Posterior Pituitary Gland and the Role of Oxytocin and Vasopressin Flashcards
Anterior Pituitary secretes __ principle hormones
6
Posterior pituitary secretes __ principle neurohormones
2
Oxytocin and Vasopressin
Oxytocin and Vasopressin are produced by __________________
Oxytocin and Vasopressin are produced by magnocellular hypothalamic neurones.
-> they have large cell bodies, in the supraoptic and paraventricular nuclei of the hypothalamus
Magnocellular neurone key steps:
- uptake of precursors
- Synthesis of pro hormone
- Packaging int granules
- Fast axonal transport (2mm/h)
- Cleavage of pro hormone
- Storage
Features of a magnocellular neurone
- Rough ER
- Golgi complex
- Pituitary stalk
Recall the Stages in stimulus secretion coupling within an anon terminal of the post. pituitary
- Arrival of Action potential
- Depolarisation by action potential; influx of sodium ions
- Opening of Calcium Chanels
- Exocytosis of granule contents (oxytocin and Neurophysin)
- Recapture of granule membrane
- Calcium incorporation into microveiscles and/or extrusion
- Restoration of membrane potential by a NA+-K+ pump
Oxytocin
- 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
Vasopressin
- 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
Vasopressin and Oxytocin act via G protein coupled receptors
- two subtypes of vasopressin receptor, V1 and V2
Activation of Oxytocin and type V1 vasopressin receptors…..
receptor promotes smooth muscle contraction by activation of PHOSPHATE C pathway:
increasing PI turnover -> mobilising intracellular calcium
Activation of Vasopressin receptor V2
this increases water permeability of renal tubes.
Mediated by the action of ADENYLATE CYCLASE and cAMP- DEPENDANT PROTEIN KINASES
Vasopressin secretion control:
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
VASOPRESSIN (ADH): at normal concentrations its actions …..
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
At HIGHER concentrations ADH acts on blood vessels and causes…
Constriction of blood vessels
raise blood pressure
What are the consequences of excessive vasopressin secretion?
Fluid retention and Decreased plasma osmolarity
What is the treatment for excessive Vasopressin secretion?
> limit fluid intake (500ml/day)
>tumour removal?
What can cause Insufficient Vasopressin secretion?
occurs after head injury/ damage to the hypothalamic-posterior-pituitary/ rare mutation preventing vasopressin production.
Consequences of insufficient vasopressin secretion
- Diabetes Insipidus
- Copious, dilute, urine
- Intense thirst
- Polydipsia (thirst)
Treatment for insufficient vasopressin secretion?
nasal spray or injection of a synthetic analogue of vasopressin
Oxytocin
Suckling induces release of oxytocin-> neuroendocrine reflex
Neuroendocrine reflex controlling milk ejection
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
What kind of reflex is the release of oxytocin
Condition reflex- top baby cry. STRESS can inhibit milk ejection reflex.
Milk Ejection involves a neuroendocrine _________________ loop
Positive feedback
Describe the positive feedback loop of milk ejection
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
What else cause oxytocin secretion?
Tactile stimulation of female reproductive tract i.e. vagina, cervix.
especially in late pregnancy / post part period
contraction of uterus = expulsion of foetus
