pituitary Flashcards
neurohypophysis: explain the principle features of the posterior pituitary; list the neurohypophysial hormones, recall how their chemical structures differ and explain their homeostatic control; explain the synthesis, storage, release and physiological action of these hormones and how dysregulation may present
what is the posterior pituitary also called
neurohypophysis
neurohypophysis origin
downgrowth from neural tissue
principle features of posterior pituitary
very long neurones originating in hypothalamus, projecting to posterior, and releasing vasopressin and oxytocin
where are neuronal cell bodies located
in supraoptic and paraventricular nuclei
supraoptic neurone pathway
leave hypothalamic supraoptic nuclei, pass through median eminence, terminate in neurohypophysis
what two types of supraoptic neurone are there
vasopressinergic or oxytocinergic
sites of vesicle storage of hormones in supraoptic neurones
Herring bodies
magnocellular paraventricular neurone pathway
leave paraventricular nuclei, terminate in neurohypophysis
parvocellular neurone pathway
leave paraventricular nuclei, terminate in the median eminence (similar to antrerior pituitary) or other parts of the brain
what influences ACTH secretion, and what is the outcome
vasopressin and CRH influence ACTH secretion, which influences cortisol
what two types of paraventricular neurone are there
vasopressinergic or oxytocinergic
vasopressin and oxytocin: differences and significance
are nonapeptides (derived from 9 amino acids): vasopressin: Phe and Arg; oxytocin: Ile and Leu; as very similar, some overlap in functions
purpose of signal peptide
signal peptide allows pre-prohormone from cytoplasm to be recognised by Golgi
fate of signal peptide
cleaved off to leave prohormone
formation of hormone from prohormone
as vesicles move along neurone axon, enzyme splits prohormone into hormone (vasopressin or oxytocin) and other sections
other sections of cleaved prohormone besides vasopressin
neurophysin and glycopeptide
other sections of cleaved prohormone besides oxytocin
different form of neurophysin, no glycopeptide
neurophysin purpose
carrier protein which protects and carries hormone to posterior pituitary before secreted
name the two different vasopressin receptors
V1 and V2
V1 vasopressin receptors: intracellular response
intracellular calcium increase to produce cellular response
where is V1a present
arterial/arteriolar smooth muscle
V1a function
acts as a vasoconstrictor
where is V1b present
anterior pituitary in corticotrophs
V1b function
ACTH production
V2 vasopressin receptors: intracellular response
increase in cAMP to produce cellular response
where is V2 present
collecting duct cells
V2 function
water reabsoroption as it is anti-diuretic hormone
V2 as anti-diuretic hormone: binding to PKA
V2 binds to receptor → activates G protein → increases activity of adenyl cyclase → more cAMP produced → activates PKA
V2 as anti-diuretic hormone: PKA to aquaporins
PKA → increase synthesis and movement of AQP2 aquaporin channels towards impermeable apical membrane → insertion → AQP3 and AQP4 on basolateral membrane allow water movement into plasma
what mechanism allows water to pass through AQP2
osmotic gradient from lumen to blood
physiological action of oxytocin: major therapeutic effects
uterus, mammary gland (myoepithelial cells)
physiological action of oxytocin: minor therapeutic effects
cardiovascular system (vasodilation), kidney (anti-diuresis), CNS (maternal behaviour)
oxytocin on mammary gland
act on breast during lactation; myeopithelial cells contract to promote milk ejection (similar to prolactin)
oxytocin on uterus
act on uterus at parturition; myometrial cells contract to promote delivery of baby; rhythmic contaction to increase local prostanoid production and dilute cervix
what is oxytocin suppresed by
progesterone
what is oxytocin enhanced by
oestrogen
when does the uterus become more sensitive to oxytocin
as weeks of gestation increase (delivery nears)
by which mechanism does the uterus become more sensitive to oxytocin
positive feedback
oxytocin on “tend and befriend” female behavioural response to stress
tend: protect and care for children; befriend: seek out and receive social support; inhibitory effects on endocrine stress response; more stimulated by oestrogen than testosterone
major clinical uses of oxytocin
induction of labour at term, prevention treatment of post-partum haemorrhage, faciliation of milk let-down, social responsiveness e.g. autism
control of vasopressin: increased plasma osmolarity
increased plasma osmolarity → osmoreceptors in hypothalamus shrink → stimulates neurones which stimulate vasopressin secretion and feeling of “thirst” → vasopressin moves down to kidney → increases water reabsorption → plasma osmolariy returns to normal range
control of vasopressin: blood pressure
low arterial BP (dehydration or hemmorhage) → baroreceptor (which inhibits vasopressin release) firing rate drops → inhibition reduced → vasopressin secretion increases → acts as vasoconstricor so blood pressure restored
control of oxytocin: milk ejection
neuroendocrine reflec arc same as prolactin; neural afferent (to brain) neurones; endocrine efferent (to peripheral tissue) secretion
oxytocin dysregulation
not essential; parturition and milk ejection effects induced/replaced by other means
vasopressin dysregulation: low levels
constant loss of water (drink and urinate a lot) which causes diabetes insipidus
where does diabetes insipidus come from: cranial
lack of vasopressin production
where does diabetes insipidus come from: nephrogenic
kidneys resistant to vasopressin
signs of diabetes insipidus
polydipsia (increased thirst), polyuria (large volumes of urine), hyper-osmolar (very dilute) urine
vasopressin dysregulation: excessive levels
caused by certain tumours; too much water reabsorbed (syndrome of inappropriate ADH)
