5) Posterior Pituitary Flashcards
What hormone does the supraoptic nuclei produce?
ADH
What hormone does the paraventricular nuclei produce?
Oxytocin
Where are nuclei located?
Within a specific region of the hypothalamus
How many amino acids compose the posterior pituitary hormones?
9 amino acids (nonapeptide)
What does the disulfide bridge form in posterior pituitary hormones?
Ring structure
Which amino acids differentiate ADH and oxytocin?
- Amino acid 3 and amino acid 8
- They are structurally similar
How is oxytocin and ADH structure similar? How is it different?
- The linear structure of amino acids is similar
- The three-dimensional structure is different
Oxytocin causes the contraction of two types of muscle cells. What are they?
- Myoepithelial cells of the alveoli of the breast
- Smooth muscle cells of the uterus during labour
What are the two primary functions of ADH?
- H2O retention by the kidney
- Contraction of blood vessels (arterioles)
What are the two physiological systems regulated by ADH?
- Osmotic
- Pressure/volume
Are the two receptors utilized for operating different physiological systems by ADH the same, or different?
They are different
Where is the V1a receptor located?
- Vascular smooth muscle
- Platelets
- Hepatocytes
- Myometrium
Where is the V1b receptor located?
Anterior pituitary
Where is the V2 receptor located?
- Basolateral membrane collecting tubule
- Vascular endothelium
- Vascular smooth muscle
What is the function of the V1a receptor in vascular smooth muscle?
- Vasoconstriction
- Myocardial hypertrophy
What is the function of the V1a receptor in platelets?
Platelet aggregation
What is the function of the V1a receptor in hepatocytes?
Glycogenolysis
What is the function of the V1a receptor in the myometrium?
Uterine contractions
What is the function of the V1b receptor in the anterior pituitary?
ACTH release
What is the function of the V2 receptor in the basolateral membrane of the collecting tubule?
Insertion of AQP2 water channels into apical membrane
What is the function of the V2 receptor in the vascular smooth muscle?
Vasodilation
What is the water-retaining hormone in mammals?
Vasopressin
What are the primary regulators of osmolarity?
- Thirst
- Vasopressin
What does the osmostat control?
- Conservation of water
- Regulation of sodium concentrations in plasma
What physiological system is modulated by the regulation of sodium concentration in plasma? What does that involve?
- Pressure-volume
- Involves baroreceptors, the renin-angiotensin system, and aldosterone
How does the relationship for the conservation of water differ from the mechanism for the regulation of sodium?
- Water is simple
- Sodium is complicated
What receptors detect changes in blood plasma? Where are they located?
Osmoreceptors located in the hypothalamus
How do osmoreceptor-containing cells vary when the blood is too dilute?
Cell expands
How do osmoreceptor-containing cells vary when the blood is too concentrated? What signal is sent?
- Cell contracts
- Contraction sends neural signal to supraoptic nuclei to release vasopressin
What are the homeostatic effects in response to hypotonicity?
- Decrease in natriuresis (Na+ retention)
- Increase in salt appetite
- Decrease in thirst
- Decrease in vasopressin
- Dilute urine produced
What are the homeostatic effects in response to hypertonicity?
- Increase in natriuresis (Na+ excretion)
- Decrease in salt appetite
- Increase in thirst
- Increase in vasopressin (water retention)
- Concentrated urine produced
What characterizes a hypotonic cell?
- Cell expands
- Net flow of water into the cell
What characterizes a hypertonic cell?
- Cell shrinks
- Net flow of water out of the cell
How does VR-OAC receptor sense a change in osmolarity?
- Receptor is membrane-bound, and the intracellular domain is linked to the cytoskeleton
- As the cell expands or contracts, it pulls against the cytoskeleton, changing the conformation of the receptor
What factors stimulate the posterior pituitary to synthesize vasopressin?
- Angiotensin II
- Hyperosmolarity
- Decreased atrial receptor firing
- Sympathetic stimulation
What regions of the nephron does vasopressin influence?
- Distal tubule
- Collecting duct
How does vasopressin cause the insertion of new water channels?
- Vasopressin binding from its receptor activates a GPCR (Gas), which increases cyclic AMP
- Results in the insertion of aquaporins on the luminal side of the tubule
Differentiate the functions of aquaporin 2 and aquaporin 3.
- Aquaporin 2 (lumen of the collecting duct): water flows into the cell
- Aquaporin 3 (blood): water flows out of the cell into the blood
Which aquaporin subtype is constitutive?
3
What are the effects of an increased quantity of aquaporins in the kidney tubules?
- Production of concentrated urine
- Decreased urine output (antidiuresis)
What is the function of the thirst reflex?
- Replacement of water in the body (loss of water through urine, breathing, sweat)
- Defense mechanism
What changes trigger the thirst mechanism?
Changes in osmolarity or volume
What is the thirst mechanism strongly triggered by, specifically?
- Hypovolemia
- Decrease in blood pressure
Generally, people ingest (not enough/too much) fluid.
too much
Does osmolarity trigger the thirst reflex often?
- No, changes are usually too small to trigger thirst
- Must be 1 to 2% below basal level to trigger thirst
How is water balance regulated under normal conditions?
Water excretion
What occurs to plasma osmolarity and blood volume during dehydration? What occurs as a result?
- Osmolarity increases
- Blood volume decreases
- Vasopressin release to retain water produces concentrated urine
What occurs to plasma osmolarity when water is in excess? What occurs as a result?
- Small decrease in osmolarity
- Less vasopressin release, resulting in dilute urine
What occurs to the osmostat during pregnancy? What occurs to volume and pressure?
- Osmostat is reset (less sensitive)
- Volume and pressure is reset
- Total body water increases 7 to 8 L as a result of vasodilation
How is the expanded volume during pregnancy sensed? What occurs to vasopressin?
- Expanded volume is sensed as “normal”
- Vasopressin responds to the reset point
What enzyme does the placenta produce that is related to vasopressin?
- Vasopressinase
- Catabolizes vasopressin that is released, allowing for the control of the quantity of vasopressin
How does total body water vary with age? What are the elderly more susceptible to?
- Total body water declines to as low as 50%
- Susceptible to both hypo- and hypernatremia
How does vasopressin and kidney function change in the elderly?
- Decrease in kidney filtration rate
- Collecting duct is less responsive to vasopressin
How does the thirst mechanism vary with plasma osmolarity?
- Within a physiological range, thirst is NOT stimulated, until a certain set-point is reached
- Afterwards, thirst increases linearly with plasma osmolarity
How does the urine osmolarity vary with vasopressin release?
- As osmolarity increases in the body, there is greater vasopressin release, and increased concentrated body urine (linear relationship)
- At a certain point, we cannot make urine more concentrated
How does urine volume vary with urine osmolarity?
As osmolarity increases, urine volume decreases (to its minimal level)
How does alcohol affect urine output? Why?
- Ethanol depresses the hypothalamic osmoreceptor
- Results in less vasopressin
- Increased urine output leading to dehydration
Define diabetes insipidus.
The excretion of a large volume of urine (diabetes) that is hypotonic, dilute, and tasteless (insipid)
What are the four causes of diabetes insipidus?
1) Lack of vasopressin
2) Lack of response to vasopressin in the kidney
3) Rapid metabolism of vasopressin
4) Polydipsia
What may cause a lack of vasopressin?
- Trauma
- Tumour
What may cause a lack of response to vasopressin in the kidney?
- Receptor defect
- Aquaporin defect
What life stage is particularly associated with the rapid metabolism of vasopressin?
- Vasopressin is less available during pregnancy
- Transient diabetes insipidus
What is polydipsia?
Individual drinks too much, and excess urine is normal
How is polydipsia differentiated from other diabetes insipidus?
- Water deprivation test
- Polydipsic patients produce concentrated urine
- Patients with diabetes insipidus do not
How is a lack of response to vasopressin differentiated from a lack of vasopressin?
- Inject individuals with vasopressin
- If they produce concentrated urine, they have a deficiency in vasopressin
- If they do not, they have a downstream problem (rapid metabolism or cells don’t respond)
What kind of receptors signal vasopressin release based on volume and pressure changes?
- Aortic arch baroreceptor
- Carotid sinus baroreceptor
Is vasopressin released when there is an increase or decrease in volume and pressure?
- 8% decrease in volume
- 5% decrease in pressure
What other systems respond to a decrease in volume and pressure?
- Sympathetic stimulation
- Angiotensin II
Where does extrapituitary synthesis of oxytocin occur?
- Ovaries (corpus luteum), which is involved in luteolysis
- Uterus in some species
What regulates oxytocin?
Suckling stimuli
What are the critical function of oxytocin?
- Lactation (milk let-down)
- Contraction of smooth muscle around the uterus during parturition
Where are oxytocin receptors located?
- Glandular cells in the mammary alveoli
- Myoepithelial layers in the mammary ducts
How is the tactile response of the nipple for the secretion of milk regulated by the CNS and oxytocin?
- Stimulation of the nipple results in a neuroreflex arc, which feedsback to the paraventricular nuclei in the hypothalamus
- Oxytocin contents are released from the posterior pituitary, and bind to receptors on the myoepithelial cells, which contract
How is the uterine myometrium during pregnancy? Which hormones regulate this state?
- Relaxed during pregnancy
- Progesterone from the placenta/corpus luteum
- Relaxin from the cervix
What do the smooth muscles of the uterus become responsive to as parturition approaches?
- Oxytocin
- Increased numbers of receptors
- Formation of gap junctions (synchronous contraction)
What does oxytocin work in conjunction with during parturition?
Prostaglandin F2a
How does oxytocin secretion increase during labour?
- Burst of oxytocin secretion by the posterior pituitary
- Released in a pulsatile manner
Oxytocin release during parturition is triggered by the ________ reflex.
Fergusson reflex
What is the Fergusson reflex?
- Self-sustaining cycle of uterine contractions initiated by pressure at the cervix or vaginal walls
- Positive feedback loop
What primes postpartum secretion of oxytocin?
Changes in steroid hormone concentrations during parturition
How do steroid hormone concentrations change during parturition?
- Increase in estradiol
- Decrease in progesterone
How does oxytocin function at the CNS level?
- Maternal behaviour
- Sexual arousal
How does oxytocin function in the regulation of the reproductive cycle?
Luteolysis
What is luteolysis? When does it occur?
- If fertilization does not occur
- The corpus luteum undergoes luteolysis and a new reproductive cycle is initiated
How does oxytocin regulate luteolysis?
- Oxytocin produced locally in the ovary binds to oxytocin receptors
- Endometrial synthesis of PGF2a
- Vasoconstriction of endometrial arterioles
- Endometrial necrosis and sloughing off
What are the two functions of the PGF2 receptor-complex?
- Increases intracellular calcium concentrations, causing apoptotic effects
- Activates PKC, which inhibits progesterone synthesis
How is oxytocin implicated in social behaviour?
- In females, bonding with new born or mate
- “The hormone of love and bonding”
What animal study reinforced the idea that oxytocin is implicated in social behaviour?
- Animal with high concentration of oxytocin receptors: mate for life
- Animal with low concentration of oxytocins receptors: bang then bye-bye
How is oxytocin related to anxiety and stress?
- Oxytocin decreases anxiety and stress
- By inhibiting anxiety and stress, many maladaptive behaviours are inhibited as well, but this is not a magic solution
What is thought to occur to oxytocin release when there is healthy social behaviour? What about during a deficit of social behaviour? Is this thought to be true?
- Healthy social behaviour results in high oxytocin release
- Deficit in social behaviour results in low oxytocin release
- This is NOT likely to be true
What life cycles is oxytocin thought to be implicated in?
- Parturition: uterine contractions
- Lactation: milk let-down
- Childhood: maternal behaviour
- Adulthood: bonding, sexual behaviour, aggressiveness
